Low: VirtualDomain: cache domain emulator after start

If libvirt goes down, we want to continue to be able to
monitor the domain. We can do this by looking at the domain's
emulator process. Typically we can get this emulator from the
domain's configuration file, but it is possible that the
configuration file will not have an emulator specified. In this
case libvirt uses whatever the default emulator is. To reliably
be able to monitor domains after libvirt goes down, we need to cache
the emulator path in a temporary file so we can retrieve it later.