Ovmf/Xen: move XenBusDxe to abstract XENIO_PROTOCOL

Ovmf/Xen: move XenBusDxe to abstract XENIO_PROTOCOL

While Xen on Intel uses a virtual PCI device to communicate the
base address of the grant table, the ARM implementation uses a DT
node, which is fundamentally incompatible with the way XenBusDxe is
implemented, i.e., as a UEFI Driver Model implementation for a PCI
device.

Contributed-under: TianoCore Contribution Agreement 1.0
Acked-by: Laszlo Ersek <lersek@redhat.com>
Reviewed-by: Anthony PERARD <anthony.perard@citrix.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@16973 6f19259b-4bc3-4df7-8a09-765794883524