Fix xenstore-rm race condition

If an entry is already gone from xenstore, any other xenstore-rm will
raise.

When we make calls to nova-agent, we wait for the response in Xenstore.
After a timeout period, we attempt to remove it.  If the DomID has
changed (due to a Windows VM rebooting at the right time via sysprep,
etc)... or if the agent picked up the command and didn't respond in a
timely fashion, the removal attempt will fail.  It would be normal for
the entry to be gone in this case.

This fixes the plugin to ignore when the entry is already
deleted.

Fixes bug 1193720

Change-Id: I8585a9f147cad3c5abe531620872b475eb4128c9
Merge "Fix xenstore-rm race condition"