Prepare xen_be_send_notify to be shared with frontends:
 * xen_be_send_notify -> xen_pv_send_notify

Signed-off-by: Emil Condrea <emilcondrea@gmail.com>
Signed-off-by: Stefano Stabellini <sstabellini@kernel.org>
Signed-off-by: Quan Xu <xuquan8@huawei.com>
Acked-by: Anthony PERARD <anthony.perard@citrix.com>