qemuMonitorAddNetdev: Convert to the native JSON props object

qemuMonitorAddNetdev: Convert to the native JSON props object

Now that all code paths generate JSON props we can remove the conversion
to command line arguments and back in the monitor code.

Note that the test which is removed in this commit will be replaced by a
stronger testsuite later.

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