Now that we disable any receiver whose queue is full, we do not require
senders to handle a zero return by supplying a sent callback.

This is a second step towards allowing can_receive() handlers to return
true even if no buffer space is available.

Signed-off-by: Mark McLoughlin <markmc@redhat.com>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>