Issue #16384: avoid lspci warnings on OpenVZ containers

This is a simple way not to get the warnings about lspci and dmidecode on OpenVZ containers.
Please note that I don't have currently a full development setup for salt, but I did test it by backporting
the entire `_virtual()` function from this commit to a running  OpenVZ container minion (saltversion: 2014.1.10)

I did not find matches under `tests/` for this (didn't look that hard)

From the setup.py, I gather that salt is for python >= 2.5, so that the use of sets is ok

Finally, I do realize there's currently a freeze and am not trying to rush this in 2014.7 (hope I didn't get the branch wrong).