(FACT-715) Fix LXC detection code for OpenVZ.

This patch fixes issue FACT-715. It works for me and it should not break anything.
I was (and still am) not sure how exceptions are hadled in facter, being unfamiliar with the code base, so I wanted to keep a minimal impact. But if there is nowhere else the exception can be rescued, the if is indeedd redundant. I will update my patch ASAP.