json: make it easier to type-check when getting from object

json: make it easier to type-check when getting from object

While working in qemu_monitor_json, I repeatedly found myself
getting a value then checking if it was an object.  Add some
wrappers to make this task easier.

* src/util/virjson.c (virJSONValueObjectGetByType)
(virJSONValueObjectGetObject, virJSONValueObjectGetArray): New
functions.
(virJSONValueObjectGetString, virJSONValueObjectGetNumberInt)
(virJSONValueObjectGetNumberUint)
(virJSONValueObjectGetNumberLong)
(virJSONValueObjectGetNumberUlong)
(virJSONValueObjectGetNumberDouble)
(virJSONValueObjectGetBoolean): Simplify.
(virJSONValueIsNull): Change return type.
* src/util/virjson.h: Reflect changes.
* src/libvirt_private.syms (virjson.h): Export them.
* tests/jsontest.c (testJSONLookup): New test.

Signed-off-by: Eric Blake <eblake@redhat.com>