test-cutils: Clean up qemu_strtoul() result checks

Use unsigned comparisons to check the result of qemu_strtoul() and
strtoull().

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Message-Id: <1487708048-2131-5-git-send-email-armbru@redhat.com>
