autotest: Refactor how XMLRPC structs are constructed

Refactor parameters to the shill xmlrpc server so that
marshalling/demarshalling happens automagically and constructors become
useful again.  This makes usage between the security configurations and
the basic xmlrpc parameters consistent.  Since this facility might be be
more broadly useful, check the base class in outside WiFi specific code.

TEST=Ran passing run_remote_tests.sh for:
wifi_check11b
wifi_checkWEP40
wifi_checkWPA2
wifi_dynamicWEP
network_WiFi_BgscanBackoff

BUG=None.

Change-Id: Id47c73f33d8b328338c44107f1c40b4dc75bf338
Reviewed-on: https://gerrit.chromium.org/gerrit/59576
Tested-by: Christopher Wiley <wiley@chromium.org>
Reviewed-by: mukesh agrawal <quiche@chromium.org>
Commit-Queue: Christopher Wiley <wiley@chromium.org>
9 files changed