Add support for storage live mirgation in UI

[stgraber]

It was reported that the web UI supports both live migration and storage pool switches, but when attempting a storage pool switch on a cluster with a running VM, it doesn't ask for a new target and therefore fails to trigger storage live migration.

It shouldn't be too hard to extend the form to detect that the instance is running, is a VM and that we're on a cluster, if all of those conditions are met, then show the target host drop-down.