virtio-blk: fix use-after-free while handling scsi commands

The scsi passthrough handler falls through after completing a
request into the failure path, resulting in a use after free.

Reproducible by running a guest with aio=native on a block device.

Reported-by: Stefan Priebe <s.priebe@profihost.ag>
Signed-off-by: Avi Kivity <avi@redhat.com>
Signed-off-by: Stefan Hajnoczi <stefanha@linux.vnet.ibm.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>

hw/virtio-blk.c

@@ -254,6 +254,7 @@ static void virtio_blk_handle_scsi(VirtIOBlockReq *req)
 
     virtio_blk_req_complete(req, status);
     g_free(req);
+    return;
 #else
     abort();
 #endif