[fedora-virt] Simplest way to notice a virtual machine shutdown?

[Tom Horsley]

I've got one virtual machine that provides some services used
by another virtual machine. What's the simplest way to notice
when one of the machines goes down and automagically tell
the other to shutdown?

Can I get notifications of some kind from libvirt or
udev (maybe the vnet interface going away) or is it
simpler to just poll every so often to see which ones
are up?