OvmfPkg/XenBusDxe: Add Event Channel Notify.

OvmfPkg/XenBusDxe: Add Event Channel Notify.

This first function is used to notify the other side that there is
something to do. The other side is another Xen domain.

Change in V4:
- Replace the license by the commonly used file header text.

Change in V3:
- Return error code from hypercall instead of ASSERT for
  XenEventChannelNotify
- moving event_channel.h to this patch.

Change in V2:
- file header
- coding style
- adding comment to functions
- Licenses

License: This patch adds event_channel.h which is under MIT licence.
Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Anthony PERARD <anthony.perard@citrix.com>
Acked-by: Jordan Justen <jordan.l.justen@intel.com>
git-svn-id: https://svn.code.sf.net/p/edk2/code/trunk/edk2@16265 6f19259b-4bc3-4df7-8a09-765794883524