(FACT-484) check dmi sysfs file for readability instead of just existence

Retrieving the virtual fact without root privileges would yield
warning messages due to missing permissions on
/sys/firmware/dmi/entries/1-0/raw (at least on certain versions of Debian).

The readable? query makes more sense than exists? at this point, anyway.
Merge pull request #690 from ffrank/ticket/facter-2/FACT-484-virtual-avoid-permission-warnings

Ticket/facter 2/fact 484 virtual avoid permission warnings