https://bugzilla.redhat.com/show_bug.cgi?id=1195882

Original commit id 'cbde3589' indicates that the cache file would be
discarded if either the QEMU binary or libvirtd 'ctime' changes; however,
the code only discarded if the QEMU binary time didn't match or if the
new libvirtd ctime was later than what created the cache file.

Since many factors come into play with 'ctime' adjustments (including
perhaps turning back the hands of time), change the logic to also force
a refresh if the ctime of libvirt is different than what's in the cache.