909495: Virt-only subscriptions are not exportable

Virt only patch for 0.7.13-hotfix branch

TEST
--------
git checkout 0.7.13-hotfix
curl https://github.com/candlepin/candlepin/pull/179.patch | git am
buildconf/script/deploy
buildr spec features="virt_only"

Then do a full unit and spec test run. The above is a quick sanity test. If it passes, move on to more extensive testing. Otherwise, just stop there.

Once this is verified it needs to merge into 0.7.13-hotfix branch.