qdev-monitor: Unref device when device_add fails

qdev_device_add() leaks the created device upon failure.  I suspect this
problem crept in because qdev_free() unparents the device but does not
drop a reference - confusing name.

Cc: qemu-stable@nongnu.org
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Signed-off-by: Andreas Färber <afaerber@suse.de>
