OvmfPkg/XenBusDxe: Add Event Channel into XenBus protocol.

OvmfPkg/XenBusDxe: Add Event Channel into XenBus protocol.

This patch adds three event channel related functions:
- EventChannelAllocate: Allocate an event channel port that can be bind
  from a specified domain.
- EventChannelNotify: Send an event to the remote end of a channel.
- EventChannelClose: Close a local event channel port.

Change in V3:
- eventchannel, update protocol to return error code.
- expand patch description
- Add comments in the XenBus Protocol header.

Change in V2:
- coding style
- adding comment to functions
- Rename 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@16271 6f19259b-4bc3-4df7-8a09-765794883524