In an upcoming patch, I need the way to safely transfer a nested
virJSON object out of its parent container for independent use,
even after the parent is freed.

* src/util/virjson.h (virJSONValueObjectRemoveKey): New function.
(_virJSONObject, _virJSONArray): Use correct type.
* src/util/virjson.c (virJSONValueObjectRemoveKey): Implement it.
* src/libvirt_private.syms (virjson.h): Export it.
* tests/jsontest.c (mymain): Test it.
Signed-off-by: NEric Blake <eblake@redhat.com>