qemu_monitor_json: Use g_autoptr() for virJSONValue

qemu_monitor_json: Use g_autoptr() for virJSONValue

A lot of explicit free calls can be saved when virJSONValue
variables are declared with g_autoptr(). There's one caveat:
there was a slight deviation from our usual pattern such that
@cmd variable was not initialized to NULL but as the very first
step it was assigned a value using qemuMonitorJSONMakeCommand().
While this works in theory it upset my GCC-11.2 (but only when
building with -O2). So I had to initialize the variable in such
case too.

Signed-off-by: Michal Privoznik <mprivozn@redhat.com>
Reviewed-by: Tim Wiederhake <twiederh@redhat.com>