handle s390x's without vm info in sysinfo

Make hwprobe.py test mode detect missing
crucial facts.

Make parse_s390_sysinfo return a dict. Verify
that info exist (it may not on some machines, so
fail back to normal methods)
use sets to find missing facts
Merge pull request #623 from candlepin/alikins/socket_s390x_no_sysinfo

handle s390x's without vm info in sysinfo