OvmfPkg/Microvm/pcie: mPhysMemAddressWidth tweak

OvmfPkg/Microvm/pcie: mPhysMemAddressWidth tweak

microvm places the 64bit mmio space at the end of the physical address
space.  So mPhysMemAddressWidth must be correct, otherwise the pci host
bridge setup throws an error because it thinks the 64bit mmio window is
not addressable.

On microvm we can simply use standard cpuid to figure the address width
because the host-phys-bits option (-cpu ${name},host-phys-bits=on) is
forced to be enabled.  Side note: For 'pc' and 'q35' this is not the
case for backward compatibility reasons.

Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>