migration/rdma: Allow cancelling while waiting for wrid

When waiting for a WRID, if the other side dies we end up waiting
for ever with no way to cancel the migration.
Cure this by poll()ing the fd first with a timeout and checking
error flags and migration state.

Signed-off-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
Reviewed-by: Peter Xu <peterx@redhat.com>
Message-Id: <20170717110936.23314-5-dgilbert@redhat.com>
Signed-off-by: Juan Quintela <quintela@redhat.com>
