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: NStefan Priebe <s.priebe@profihost.ag>
Signed-off-by: NAvi Kivity <avi@redhat.com>
Signed-off-by: NStefan Hajnoczi <stefanha@linux.vnet.ibm.com>
Signed-off-by: NKevin Wolf <kwolf@redhat.com>
(cherry picked from commit 730a9c53)
Signed-off-by: NMichael Roth <mdroth@linux.vnet.ibm.com>

hw/virtio-blk.c

@@ -253,6 +253,7 @@ static void virtio_blk_handle_scsi(VirtIOBlockReq *req)
 
     virtio_blk_req_complete(req, status);
     g_free(req);
+    return;
 #else
     abort();
 #endif