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