qemuxml2argvtest: Don't re-parse and re-generate commandline for QAPI schema validation

qemuxml2argvtest: Don't re-parse and re-generate commandline for QAPI schema validation

The removal of the special internal flag for '-netdev' validatition now
allows us to use the same virCommand object for validation of the
schema.

Pass it into the validator instead of re-parsing and re-generating
everything.

This improved the runtime of qemuxml2argvtest by ~25% on my box.

Signed-off-by: Peter Krempa <pkrempa@redhat.com>
Reviewed-by: Erik Skultety <eskultet@redhat.com>