multifd: Only send pages when packet are not empty

We send packages without pages sometimes for sysnchronizanion.  The
iov functions do the right thing, but we will be changing this code in
future patches.

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