vhost: fix enabling vring per queue

vhost: fix enabling vring per queue

The VHOST_USER_SET_VRING_ENABLE request was sent for each queue-pair.
However, it's changed to be sent per queue in the queue-pair by QEMU
commit dc3db6ad ("vhost-user: start/stop all rings"). The change
is reasonable, as we send all other request per queue, instead of
queue-pair.

Hence we should do proper changes to adapt to the QEMU change here.
Otherwise, a segfault will be triggered when last TX queue was enabled.

Signed-off-by: Victor Kaplansky <victork@redhat.com>
Signed-off-by: Yuanhan Liu <yuanhan.liu@linux.intel.com>