Clone:
- only shut-off VMs are cloned, as the UI already required: a running
guest keeps writing while its disks are copied; the definition comes
from the persistent XML;
- MAC addresses and disk destinations are checked before anything is
allocated;
- every volume the clone creates (NVRAM and disks) is tracked; if a
later copy or the definition fails, only those are deleted, and any
that cannot be deleted are named in the error. Before, a failed clone
left its earlier copies behind.
libvirtError(msg) reports the thread's last libvirt error instead of
msg when there is one, so WebVirtCloud's own errors could show an
earlier, unrelated failure. util.OperationError, a libvirtError
subclass that keeps its message, replaces those raises in vrtManager,
instances and nwfilters; 'except libvirtError' still catches it.
