currently it is not possible to distinguish by exitcode if there
has been an error or if bdrv_check is not supported by the image
format. Change the exitcode from 1 to 63 for the latter case.

Signed-off-by: Peter Lieven <pl@kamp.de>
Reviewed-by: Eric Blake <eblake@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>