Fix memory leak in xend

Fix memory leak in xend

I found that xend's memory usage grows considerably when running a
script such as

while true; do xm new foo ; sleep 2s; xm delete foo ; sleep 2s; done

XendAPIStore maintains a list of class instances and in the case of
new/create operation, an associated VMMetrics object is stashed in the
list but never removed on delete/shutdown.  This patch removes the
instance by invoking XendBase.destroy() method.

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