In preparation for converting the generator of -netdev to generate JSON
which will be used to do the command line rather than the other way
around we need to introduce a convertor which properly configures
virQEMUBuildCommandLineJSON for the quirks of -netdev.
Signed-off-by: NPeter Krempa <pkrempa@redhat.com>
Reviewed-by: NEric Blake <eblake@redhat.com>