apt: Make not-yet-installed virt pkgs work with pkg.latest

This commit makes pkg.latest_version look at virtual packages if the
targeted package is not found with apt-cache policy. This allows for
targeted virtual packages to be installed for the first time with
pkg.latest rather than pkg.installed. Because of the nature of virtual
packages however, a virtual package will never be updated using
pkg.latest, since they don't actually have version numbers (salt uses
'1' as a version number for virtual packages). So, it's not really
recommended that one use a pkg.latest state for virtual packages, but if
they do, the states will at least not fail on the initial install
attempt.