I tagged this as bare metal because this has implications on bare metal, especially in the case where a BIOS/firmware might provide no assistance for configuring PCI BAR windows at all.

I simply added the following line to the run.sh script, in the SERENITY_COMMON_QEMU_Q35_ARGS setting:

-bios /usr/share/qemu/qboot.rom

Trying to boot with qboot on the Q35 machine yields the following crash:

0.145 [#0 init_stage2(1:1)]: VMWare SVGA @ PCI [0000:00:01:00], 0x000000200bdff120
0.145 [#0 init_stage2(1:1)]: VMWare SVGA @ PCI [0000:00:01:00]: Accepted version 2415919106
0.145 [#0 init_stage2(1:1)]: VMWare SVGA capabilities (raw 90000002):
0.149 [#0 init_stage2(1:1)]: 	Rect copy
0.149 [#0 init_stage2(1:1)]: VMWare SVGA @ PCI [0000:00:01:00]: framebuffer size 2415919106 bytes, FIFO size 2415919106 bytes @ P0000000000000000
0.172 [init_stage2(1:1)]: RegionTree: Failed to allocate anywhere: size=2415923200, alignment=4096
0.183 [#0 init_stage2(1:1)]: Failed to initialize device PCI [0000:00:01:00], due to Error(errno=12)
[init_stage2(1:1)]: ASSERTION FAILED: bucket.get_buddy_bit(index) == true
[init_stage2(1:1)]: ./Kernel/Memory/PhysicalZone.cpp:110 in AK::Optional<short int> Kernel::Memory::PhysicalZone::allocate_block_impl(size_t)
[init_stage2(1:1)]: KERNEL PANIC! :^(
[init_stage2(1:1)]: Aborted
[init_stage2(1:1)]: at ./Kernel/Arch/x86_64/CPU.cpp:35 in void abort()
[init_stage2(1:1)]: Kernel + 0x000000000162869f  Kernel::__panic(char const*, unsigned int, char const*) +0x16f
[init_stage2(1:1)]: Kernel + 0x0000000001629bb1  abort.localalias +0x35a
[init_stage2(1:1)]: Kernel + 0x0000000001629857  abort.localalias +0x0
[init_stage2(1:1)]: Kernel + 0x0000000000d2201e  Kernel::Memory::PhysicalZone::allocate_block_impl(unsigned long) [clone .localalias] +0xdfe
[init_stage2(1:1)]: Kernel + 0x0000000000d239e5  Kernel::Memory::PhysicalZone::allocate_block(unsigned long) +0x75
[init_stage2(1:1)]: Kernel + 0x0000000000d1392d  Kernel::Memory::PhysicalRegion::take_free_page() +0x15d
[init_stage2(1:1)]: Kernel + 0x0000000000ccfd1c  Kernel::Memory::MemoryManager::find_free_physical_page(bool) [clone .localalias] +0x54c
[init_stage2(1:1)]: Kernel + 0x0000000000cd551c  Kernel::Memory::MemoryManager::allocate_committed_physical_page(AK::Badge<Kernel::Memory::CommittedPhysicalPageSet>, Kernel::Memory::MemoryManager::ShouldZeroFill) [clone .localalias] +0x6c
[init_stage2(1:1)]: Kernel + 0x0000000000cd963c  Kernel::Memory::CommittedPhysicalPageSet::take_one() +0xdc
[init_stage2(1:1)]: Kernel + 0x0000000000d9b121  Kernel::Memory::SharedFramebufferVMObject::SharedFramebufferVMObject(AK::FixedArray<AK::RefPtr<Kernel::Memory::PhysicalPage> >&&, Kernel::Memory::CommittedPhysicalPageSet, Kernel::Memory::AnonymousVMObject&) [clone .localalias] +0x4f1
[init_stage2(1:1)]: Kernel + 0x0000000000da43cc  Kernel::Memory::SharedFramebufferVMObject::try_create_for_physical_range(PhysicalAddress, unsigned long) +0x37c
[init_stage2(1:1)]: Kernel + 0x0000000000360ace  Kernel::DisplayConnector::allocate_framebuffer_resources(unsigned long) [clone .localalias] +0x2fe
[init_stage2(1:1)]: Kernel + 0x00000000003669f2  Kernel::DisplayConnector::after_inserting() [clone .localalias] +0xd32
[init_stage2(1:1)]: Kernel + 0x00000000003457e8  AK::ErrorOr<AK::NonnullLockRefPtr<Kernel::QEMUDisplayConnector>, AK::Error> Kernel::DeviceManagement::try_create_device<Kernel::QEMUDisplayConnector, PhysicalAddress&, unsigned long&, Kernel::Memory::TypedMapping<Kernel::BochsDisplayMMIORegisters volatile> >(PhysicalAddress&, unsigned long&, Kernel::Memory::TypedMapping<Kernel::BochsDisplayMMIORegisters volatile>&&) +0x508
[init_stage2(1:1)]: Kernel + 0x000000000033c3b6  Kernel::QEMUDisplayConnector::must_create(PhysicalAddress, unsigned long, Kernel::Memory::TypedMapping<Kernel::BochsDisplayMMIORegisters volatile>) +0x86
[init_stage2(1:1)]: Kernel + 0x00000000018f2641  Kernel::BochsGraphicsAdapter::initialize_adapter(Kernel::PCI::DeviceIdentifier const&) [clone .localalias] +0xb81
[init_stage2(1:1)]: Kernel + 0x00000000018f5d5f  Kernel::BochsGraphicsAdapter::create(Kernel::PCI::DeviceIdentifier const&) +0x42f
[init_stage2(1:1)]: Kernel + 0x00000000018faeb2  Kernel::GraphicsManagement::determine_and_initialize_graphics_device(Kernel::PCI::DeviceIdentifier const&) [clone .localalias] +0x712
[init_stage2(1:1)]: Kernel + 0x000000000039ff82  AK::Function<void (Kernel::PCI::DeviceIdentifier const&)>::CallableWrapper<Kernel::GraphicsManagement::initialize()::{lambda(Kernel::PCI::DeviceIdentifier const&)#2}>::call(Kernel::PCI::DeviceIdentifier const&) +0x2c2
[init_stage2(1:1)]: Kernel + 0x0000000000037cfe  Kernel::PCI::Access::fast_enumerate(AK::Function<void (Kernel::PCI::DeviceIdentifier const&)>&) const +0x9de
[init_stage2(1:1)]: Kernel + 0x000000000004eac1  Kernel::PCI::enumerate(AK::Function<void (Kernel::PCI::DeviceIdentifier const&)>) +0x71
[init_stage2(1:1)]: Kernel + 0x00000000018fde8c  Kernel::GraphicsManagement::initialize() +0x66c
[init_stage2(1:1)]: Kernel + 0x00000000000034ee  Kernel::init_stage2(void*) +0xa4e