994997: Fix Unknown is_guest during firstboot.

Signal cannot be used outside of the main thread, depending on when
facts are loaded this can happen in subman, this would be the second
time we've seen it occur.

Possibly re-introduces an old bug but this is preferable to the current
situation:

https://bugzilla.redhat.com/show_bug.cgi?id=681925
Merge pull request #742 from candlepin/dgoodwin/firstboot-virt-facts

994997: Fix Unknown is_guest during firstboot.