Ovmf/Xen: add separate driver for Xen PCI device

Ovmf/Xen: add separate driver for Xen PCI device

Prepare for making XenBusDxe suitable for use with non-PCI devices
(such as the DT node exposed by Xen on ARM) by introducing a separate
DXE driver that binds to the Xen virtual PCI device and exposes the
abstract XENIO_PROTOCOL for XenBusDxe to bind against.

Contributed-under: TianoCore Contribution Agreement 1.0
Reviewed-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@16972 6f19259b-4bc3-4df7-8a09-765794883524