qemuMonitorJSONQueryRxFilterParse: Set *filter only on success

qemuMonitorJSONQueryRxFilterParse: Set *filter only on success

The qemuMonitorJSONQueryRxFilterParse() function is called to
parse the output of 'query-rx-filter' and store results into
passed virNetDevRxFilter structure. However, it is doing so in a
bit clumsy way - the return pointer is set in all cases (i.e.
even in case of error) and thus the cleanup label is more
complicated than it needs to be. With a help of g_autoptr() and
g_steal_pointer() the return pointer can be set only in case of
success - which is what callers expect anyway.

The same applies to qemuMonitorJSONQueryRxFilter().

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