ArmVirtualizationPkg/VirtFdtDxe: wire up XenBusDxe to "xen,xen" DT node

ArmVirtualizationPkg/VirtFdtDxe: wire up XenBusDxe to "xen,xen" DT node

This patchs adds support to VirtFdtDxe for the Xen DT node which
contains the base address of the Grant Table. This data is communicated
to XenBusDxe using a XENIO_PROTOCOL instance.

Contributed-under: TianoCore Contribution Agreement 1.0
Reviewed-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@16980 6f19259b-4bc3-4df7-8a09-765794883524