qemu: Prepare for testing of 'netdev_add' props via qemuxml2argvtest

qemu: Prepare for testing of 'netdev_add' props via qemuxml2argvtest

qemuxml2argv test suite is way more comprehensive than the hotplug
suite. Since we share the code paths for monitor and command line
hotplug we can easily test the properties of devices against the QAPI
schema.

To achieve this we'll need to skip the JSON->commandline conversion for
the test run so that we can analyze the pure properties. This patch adds
flags for the comand line generator and hook them into the
JSON->commandline convertor for -netdev. An upcoming patch will make use
of this new infrastructure.

Signed-off-by: Peter Krempa <pkrempa@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>