Ovmf/Xen: introduce XENIO_PROTOCOL

Ovmf/Xen: introduce XENIO_PROTOCOL

This introduces the abstract XENIO_PROTOCOL that will be used to
communicate the Xen grant table address to drivers supporting this
protocol. Primary purpose is allowing us to change the XenBusDxe
implementation so that it can support non-PCI Xen implementations
such as Xen on ARM.

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@16971 6f19259b-4bc3-4df7-8a09-765794883524