qemu_monitor_json: Follow refactor

In 7884d089 I've started to refactor qemu_monitor_json.c.
Thing is, it's current structure is nothing like the rest of our
code. The @ret variable is rewritten all the time, if()-s are
nested instead of using goto and so on.
Signed-off-by: NMichal Privoznik <mprivozn@redhat.com>