vhost-user-blk: fix the resize crash

vhost-user-blk: fix the resize crash

If the os is not installed and doesn't have the virtio guest driver,
the vhost dev isn't started, so the dev->vdev is NULL.

Reproduce: mount a Win 2019 iso, go into the install ui, then resize
the virtio-blk device, qemu crash.

Signed-off-by: Li Feng <fengli@smartx.com>
Message-Id: <20220919121816.3252223-1-fengli@smartx.com>
Reviewed-by: Raphael Norwitz <raphael.norwitz@nutanix.com>
Reviewed-by: Kevin Wolf <kwolf@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>