OvmfPkg/XenSupport: remove usage of prefetchable PCI host bridge aperture

OvmfPkg/XenSupport: remove usage of prefetchable PCI host bridge aperture

This aperture doesn't exist in QEMU-XEN and hvmloader places BARs
in arbitrary order disregarding prefetchable bit. This makes
prefetchable and non-prefetchable BARs to follow each other that's
quite likely with PCI passthrough devices. In that case, the existing
code, that tries to work out aperture boundaries by reading hvmloader
BAR placement, will report a bogus prefetchable aperture which overlaps
with the regular one. It will eventually trigger an assertion in
DXE PCI initialization code.

Do the same thing as OVMF on QEMU-KVM and pass a non-existing aperture
there. It's not necessary to pass additional allocation flags as we set
ResourceAssigned flag on the root bridge which means they will be ignored.

Reviewed-by: Anthony PERARD <anthony.perard@citrix.com>
Signed-off-by: Igor Druzhinin <igor.druzhinin@citrix.com>