vhost-scsi: The vhost backend should be stopped when the VM is not running

vhost-scsi doesn’t takes into account whether the VM is running or not in
order to decide if it should start/stop vhost backend.
This would lead to vhost backend still being active when VM's RunState
suddenly change to stopped.

An example of when this issue is encountered is when Live-Migration Pre-Copy
phase completes. As in this case, VM state will be changed to stopped (while
vhost backend is still active), which will result in
virtio_vmstate_change() -> virtio_set_status() -> vhost_scsi_set_status()
executed but vhost_scsi_set_status() will just return without stopping
vhost backend.

To handle this, change code to consider that vhost processing should be
stopped when VM is not running. Similar to how it is done in vhost-vsock
device at vhost_vsock_set_status().

Fixes: 5e9be92d7752 ("vhost-scsi: new device supporting the tcm_vhost Linux kernel module”)
Reviewed-by: Bijan Mottahedeh <bijan.mottahedeh@oracle.com>
Reviewed-by: Liran Alon <liran.alon@oracle.com>
Signed-off-by: Nir Weiner <nir.weiner@oracle.com>
Message-Id: <20190416125912.44001-2-liran.alon@oracle.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>