xend: Fix destruction of VMMetrics objects for managed domains

xend: Fix destruction of VMMetrics objects for managed domains

While plugging a xend memory lead (c/s 18030), I introduced a bug in
the case of managed domains.  Managed domains continue to exist after
shutdown and thus should not have their associated VMMetrics object
destroyed until deleted.

The bug actually manifested by a new -> start -> shutdown -> delete
sequence of operations.  Shutdown destroyed the VMMetrics object and
delete tried to do the same :-):

Signed-off-by: Jim Fehlig <jfehlig@novell.com>