vu_panic() is not guaranteed to exit the program. Return early when
errors are encountered.

Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
Message-Id: <20200921113420.154378-3-stefanha@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>