The SubmitRequest method takes a prepared XML request and
submits it to the MPF engine.
Unlike submitting requests directly to the provisioning engine,
this method expects all requests passed to it to contain an execute
node with the proper namespace and procedure supplied, even if it
is a trusted request.
If the engine returns an error and the XML string is provided,
the RequestFailed property of the object is set to true, and the
error information is returned in the XmlDocument. If an error
occurs and the engine does not return an XML string, a
ClientUnknownException is thrown.