ArmVirtualizationPkg: add platform description for Xen guests

ArmVirtualizationPkg: add platform description for Xen guests

This adds the .dsc and .fdf descriptions to build a UEFI image that
is bootable by a Xen guest on 64-bit ARM (AArch64)

Contributed-under: TianoCore Contribution Agreement 1.0
Acked-by: Laszlo Ersek <lersek@redhat.com>
Reviewed-by: Olivier Martin <olivier.martin@arm.com>
Signed-off-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Signed-off-by: Laszlo Ersek <lersek@redhat.com>
git-svn-id: https://svn.code.sf.net/p/edk2/code/trunk/edk2@16981 6f19259b-4bc3-4df7-8a09-765794883524