BindingXMLRPC bugfix: Return lastClientIface for current connection

The patch does two things:
1) Fixes a bug where lastClientIface was for the previous
   connection, and not the current one. The method to retrieve
   the last client was through a log_request LoggingMixIn
   which is invoked at the end of a response, not at the beginning.
   The patch removes the usage of that MixIn, and instead uses
   values that are updated at the beginning of the connection
2) Simplifies the 'LoggingMixin' out of the picture as its no
   longer needed

Change-Id: I36e48cc07a27f44a2b413f0e9159110404f1b0ca
Signed-off-by: Assaf Muller <amuller@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/22837
Reviewed-by: Dan Kenigsberg <danken@redhat.com>