hooking: Add convenience functions for JSON hooks

Added read_json/write_json, similar in fashion to existing *_domxml
functions

Change-Id: I163c09329cd341197251642f9850e131d9e724a2
Signed-off-by: Dima Kuznetsov <dkuznets@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/26521
Reviewed-by: Dan Kenigsberg <danken@redhat.com>