tap: properly initialize vhostfds

Only tap->vhostfd were checked net_init_tap_one(), but tap->vhostfds were
forgot, this will lead qemu to ignore all fds passed by management through
vhostfds, and tries to create vhost_net device itself. Fix by adding this check
also.

Reportyed-by: Michal Privoznik <mprivozn@redhat.com>
Cc: Michal Privoznik <mprivozn@redhat.com>
Cc: qemu-stable@nongnu.org
Signed-off-by: Jason Wang <jasowang@redhat.com>
Reviewed-by: Michal Privoznik <mprivozn@redhat.com>
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
