[Libguestfs] failure to find hostname causing fatal error

Trying to inspect an old OpenSuSE image, which has an empty /etc/HOSTNAME file. This causes check_hostname_unix() in inspect_fs_unix() to return -1 -- causing inspection to fail. Somewhat interestingly, if no files of interest were found, the function would return 0.....which I think doesn't cause problems upstream. It's somewhat related to this closed bug where the fix was to set the return code to 0 and hostname could end up null. https://bugzilla.redhat.com/show_bug.cgi?id=726739 It may be correct, in fact, to try each of the hostname checks until 1 succeeds or they all fail, and then return 0 in all cases. --Todd