xen: add event channel interface for XenDevice-s

xen: add event channel interface for XenDevice-s

The legacy PV backend infrastructure provides functions to bind, unbind
and send notifications to event channnels. Similar functionality will be
required by XenDevice implementations so this patch adds the necessary
support.

Signed-off-by: Paul Durrant <paul.durrant@citrix.com>
Reviewed-by: Anthony Perard <anthony.perard@citrix.com>
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Cc: Stefano Stabellini <sstabellini@kernel.org>
v2:
 - Added error pointers to notify and unbind