block: fix qemu-img --help invocation This fixes a bug introduced in commit ac1307ab, that caused the '--help' option to not be recognized as a valid command, and not print any help. Signed-off-by: Jeff Cody <jcody@redhat.com> Reviewed-by: Eric Blake <eblake@redhat.com> Signed-off-by: Kevin Wolf <kwolf@redhat.com>