qemu-ga: Document intentional fall through in channel_event_cb()

For clarity, and to hush up Coverity.
Signed-off-by: NMarkus Armbruster <armbru@redhat.com>
Reviewed-by: NEric Blake <eblake@redhat.com>
Reviewed-by: NMichael Roth <mdroth@linux.vnet.ibm.com>
Reviewed-by: NLuiz Capitulino <lcapitulino@redhat.com>
Signed-off-by: NMichael Roth <mdroth@linux.vnet.ibm.com>

qga/main.c

@@ -618,6 +618,7 @@ static gboolean channel_event_cb(GIOCondition condition, gpointer data)
         if (!s->virtio) {
             return false;
         }
+        /* fall through */
     case G_IO_STATUS_AGAIN:
         /* virtio causes us to spin here when no process is attached to
          * host-side chardev. sleep a bit to mitigate this