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

OvmfPkg/OvmfPkg.dec

@@ -47,6 +47,7 @@
 [Protocols]
   gVirtioDeviceProtocolGuid       = {0xfa920010, 0x6785, 0x4941, {0xb6, 0xec, 0x49, 0x8c, 0x57, 0x9f, 0x16, 0x0a}}
   gBlockMmioProtocolGuid          = {0x6b558ce3, 0x69e5, 0x4c67, {0xa6, 0x34, 0xf7, 0xfe, 0x72, 0xad, 0xbe, 0x84}}
+  gXenBusProtocolGuid             = {0x3d3ca290, 0xb9a5, 0x11e3, {0xb7, 0x5d, 0xb8, 0xac, 0x6f, 0x7d, 0x65, 0xe6}}
 
 [PcdsFixedAtBuild]
   gUefiOvmfPkgTokenSpaceGuid.PcdOvmfPeiMemFvBase|0x0|UINT32|0