qemumonitorjsontest: Extend the test for yet another monitor commands

qemumonitorjsontest: Extend the test for yet another monitor commands

So far, we're unit testing some basic functions and some (so called)
simple functions (e.g. "qmp_capabilities", "system_powerdown"). However,
there are more functions which expect simple "{'return': {}}" reply, but
takes more args to construct the command (for instance "set_link"). This
patch aims on such functions.

Signed-off-by: Michal Privoznik <mprivozn@redhat.com>