The exit code 63 (check not supported by image format) was not even
documented in the comment above the check command in the source code;
add it, as it does indeed seem useful.

Also, document all of check's exit codes in the manpage.
Signed-off-by: NMax Reitz <mreitz@redhat.com>
Reported-by: NMarkus Armbruster <armbru@redhat.com>
Reviewed-by: NEric Blake <eblake@redhat.com>
Reviewed-by: NBenoit Canet <benoit@irqsave.net>
Reviewed-by: NKevin Wolf <kwolf@redhat.com>
Signed-off-by: NStefan Hajnoczi <stefanha@redhat.com>