Apparently the most important (the only?) property required in offline
mode is "is_running". So let's patch it to return False and make sure
any other libvirt usage would result in failure.
Or maybe better simply returh False in vm.is_running, when libvirt
connection fails? But then it would not be possible to use offline mode
and have (some, probably unrelated) libvirtd running at the same time.
FixesQubesOS/qubes-issues#2008