vhost currently merges regions with contiguious virtual and physical
addresses. This breaks for vhost-user since that also needs fds to
match.
Add a vhost_ops entry to compare the fds for vhost-user only.
Cc: qemu-stable@nongnu.org
Cc: Victor Kaplansky <victork@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>