hv_xen: Remove config after shutdown was successful

If stopping an instance failed, the configuration would already be gone
and other operations depending on it (e.g. migration) would no longer
work. With this patch the configuration file is only removed once the
instance was succesfully stopped or destroyed.

Signed-off-by: Michael Hanselmann <hansmi@google.com>
Reviewed-by: Michele Tartara <mtartara@google.com>