migration: poll the cm event for destination qemu

The destination qemu only poll the comp_channel->fd in
qemu_rdma_wait_comp_channel. But when source qemu disconnnect
the rdma connection, the destination qemu should be notified.

Signed-off-by: Lidong Chen <lidongchen@tencent.com>
Reviewed-by: Juan Quintela <quintela@redhat.com>
Reviewed-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
Signed-off-by: Juan Quintela <quintela@redhat.com>