Revert "vhost: send SET_VRING_ENABLE at start/stop"

This reverts commit 3a12f322.

In case of live migration several queues can be enabled and not only the
first one. So informing backend that only the first queue is enabled is
wrong.

Reported-by: Thibaut Collet <thibaut.collet@6wind.com>
Cc: Yuanhan Liu <yuanhan.liu@linux.intel.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Reviewed-by: Yuanhan Liu <yuanhan.liu@linux.intel.com>