OvmfPkg: Introduce XenBus Protocol.

OvmfPkg: Introduce XenBus Protocol.

This protocol will be used for communication between a PV driver (like a
PV block driver) and the XenBus/XenStore.

Change in V5:
- Replace the license by the commonly used file header text.

Change in V3:
- Add disclaimer about the volatile nature of the protocol.
- Add a description on the two introduced members to the protocol.

Change in V2:
- Comment, file header
- Protocol License
- Declare xen interface version earlier
- Rename protocol from Xenbus to XenBus

Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Anthony PERARD <anthony.perard@citrix.com>
Reviewed-by: Jordan Justen <jordan.l.justen@intel.com>
git-svn-id: https://svn.code.sf.net/p/edk2/code/trunk/edk2@16262 6f19259b-4bc3-4df7-8a09-765794883524