Avoid use of free'd memory in auto destroy callback

Avoid use of free'd memory in auto destroy callback

The autodestroy callback code has the following function
called from a hash iterator

  qemuDriverCloseCallbackRun(void *payload,
                             const void *name,
                             void *opaque)
  {
    ...
    char *uuidstr = name
    ...

    dom = closeDef->cb(data->driver, dom, data->conn);
    if (dom)
        virObjectUnlock(dom);

    virHashRemoveEntry(data->driver->closeCallbacks, uuidstr);
  }

The closeDef->cb function may well cause the current callback
to be removed, if it shuts down 'dom'. As such the use of
'uuidstr' in virHashRemoveEntry is accessing free'd memory.
We must make a copy of the uuid str before invoking the
callback to be safe.

Signed-off-by: Daniel P. Berrange <berrange@redhat.com>