NACK this is putting inappropriate policy decisions into libvirt.
This may be how you want this scenario to work, but it is certainly not what every user
of libvirt will want.
How about adding some option to libvirt so that the users can choose policies?
If the storage is fubar & the management application wants to
still start the guest, they should change the XML config appropriately.
I think that providing options is user-friendly rather than asking users to edit xml by
hand after the storage is broken.
Seiji