tcm_vhost: Wait for pending requests in vhost_scsi_flush()

Unlike tcm_vhost_evt requests, tcm_vhost_cmd requests are passed to the
target core system, we can not make sure all the pending requests will
be finished by flushing the virt queue.

In this patch, we do refcount for every tcm_vhost_cmd requests to make
vhost_scsi_flush() wait for all the pending requests issued before the
flush operation to be finished.

This is useful when we call vhost_scsi_clear_endpoint() to stop
tcm_vhost. No new requests will be passed to target core system because
we clear the endpoint by setting vs_tpg to NULL. And we wait for all the
old requests. These guarantee no requests will be leaked and existing
requests will be completed.

Signed-off-by: Asias He <asias@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>