vhost-user: allow slave to send fds via slave channel

Introduce VHOST_USER_PROTOCOL_F_SLAVE_SEND_FD protocol
feature to allow slave to send at most 8 descriptors
in each message to master via ancillary data using the
slave channel.

Suggested-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Tiwei Bie <tiwei.bie@intel.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>