xenapi: skip metadata updates when VM not found

There is a race condition where change_instance_metdata requests still
get to the compute nodes when the VM has a task in operation that means
the VM is no longer present. This change ensures that we skip the
metadata updates when this occurs.

Currently, if this happens, the long running task, like rebuild or
resize will fail because the VM task_state=None and is put into ERROR.
After this change, it will silently fail, and the updated metadata will
be written when the VM gets re-created as part of the long running
operation.

Fixes bug 1207238
Change-Id: I75e1f93e34d3b3ab93a8e8104fd64224f72d7309
Merge "xenapi: skip metadata updates when VM not found"