vhost: Check for valid vdev in vhost_backend_handle_iotlb_msg

vhost: Check for valid vdev in vhost_backend_handle_iotlb_msg

Not checking this can lead to invalid dev->vdev member access in
vhost_device_iotlb_miss if backend issue an iotlb message in a bad
timing, either maliciously or by a bug.

Reproduced rebooting a guest with testpmd in txonly forward mode.
 #0  0x0000559ffff94394 in vhost_device_iotlb_miss (
     dev=dev@entry=0x55a0012f6680, iova=10245279744, write=1)
     at ../hw/virtio/vhost.c:1013
 #1  0x0000559ffff9ac31 in vhost_backend_handle_iotlb_msg (
     imsg=0x7ffddcfd32c0, dev=0x55a0012f6680)
     at ../hw/virtio/vhost-backend.c:411
 #2  vhost_backend_handle_iotlb_msg (dev=dev@entry=0x55a0012f6680,
     imsg=imsg@entry=0x7ffddcfd32c0)
     at ../hw/virtio/vhost-backend.c:404
 #3  0x0000559fffeded7b in slave_read (opaque=0x55a0012f6680)
     at ../hw/virtio/vhost-user.c:1464
 #4  0x000055a0000c541b in aio_dispatch_handler (
     ctx=ctx@entry=0x55a0010a2120, node=0x55a0012d9e00)
     at ../util/aio-posix.c:329

Fixes: 020e571b8b ("vhost: rework IOTLB messaging")
Signed-off-by: Eugenio Pérez <eperezma@redhat.com>
Message-Id: <20210129090728.831208-1-eperezma@redhat.com>
Acked-by: Jason Wang <jasowang@redhat.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>