drm/i915/gvt: Emulate PCI expansion ROM base address register

Our vGPU doesn't have a device ROM, we need follow the PCI spec to
report this info to drivers. Otherwise, we would see below errors.

Inspecting possible rom at 0xfe049000 (vd=8086:1912 bdf=00:10.0)
qemu-system-x86_64: vfio-pci: Cannot read device rom at 00000000-0000-0000-0000-000000000001
Device option ROM contents are probably invalid (check dmesg).
Skip option ROM probe with rombar=0, or load from file with romfile=No option rom signature (got 4860)

I will also send a improvement patch to PCI subsystem related to PCI ROM.
But no idea to omit below error, since no pattern to detect vbios shadow
without touch its content.
0000:00:10.0: Invalid PCI ROM header signature: expecting 0xaa55, got 0x0000

Signed-off-by: Changbin Du <changbin.du@intel.com>
Signed-off-by: Zhenyu Wang <zhenyuw@linux.intel.com>