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>
