qemu: Add libvirt version check to refresh capabilities algorithm
    
    Rather than an algorithm based solely on libvirtd ctime to refresh the
    capabilities add the element of the libvirt build version into the equation.
    Since that version wouldn't be there prior to this code being run - don't
    fail on reading the capabilities if not found. In this case, the cache
    will always be rebuilt when a new libvirt version is installed.