OvmfPkg/XenSupport: turn off address decoding before BAR sizing

OvmfPkg/XenSupport: turn off address decoding before BAR sizing

On Xen, hvmloader firmware leaves address decoding enabled for
enumerated PCI device before jumping into OVMF. OVMF seems to
expect it to be disabled and tries to size PCI BARs in several places
without disabling it which causes BAR64, for example, being
incorrectly placed by QEMU.

Fix it by disabling PCI address decoding explicitly before the
first attempt to size BARs on Xen.

Signed-off-by: Igor Druzhinin <igor.druzhinin@citrix.com>
Acked-by: Laszlo Ersek <lersek@redhat.com>
Acked-by: Anthony PERARD <anthony.perard@citrix.com>