OHAI-525: reverting part of OHAI-525

dmidecode isn't necessarily installed by default (fedora 20), while
the existing /proc/modules detection works fine.  i don't see why we
only need one mechanism, so try both and use which one works.
add back in spec for /proc/modules and xen
Merge pull request #280 from opscode/lcg/revert-OHAI-525-part

OHAI-525: reverting part of OHAI-525