gluster: Plug memory leaks in qemu_gluster_parse_json()

To reproduce, run

    $ valgrind qemu-system-x86_64 --nodefaults -S --drive driver=gluster,volume=testvol,path=/a/b/c,server.0.type=xxx

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Reviewed-by: Niels de Vos <ndevos@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
