xend: less noise in xend-debug.log on HVM shutdown

xend: less noise in xend-debug.log on HVM shutdown

Shutting down a hvm, xend-debug.log always shows:

Unhandled exception in thread started by=20
Traceback (most recent call last):
  File "//usr/lib64/python/xen/xend/image.py", line 549, in
  _sentinel_watch
    self._dmfailed(message)
  File "//usr/lib64/python/xen/xend/image.py", line 491, in _dmfailed
    xc.domain_shutdown(self.vm.getDomid(), DOMAIN_CRASH)
xen.lowlevel.xc.Error: (3, 'No such process')

Signed-off-by: Kouya Shimura <kouya@jp.fujitsu.com>