qom: if @instance_size==0, assign size of object to parent object size

qom: if @instance_size==0, assign size of object to parent object size

QOM documentation states that for objects of type with @instance_size == 0 size
will be assigned to match parent object's size. But currently this feauture is
not implemented and qemu asserts during creation of object with zero instance_size.

Set appropriate value for type instance_size during type_class_init() call.
object_initialize_with_type() must call type_class_init() before asserting
type->instance_size, and object_new_with_type() must call type_class_init() before
object allocation.

Signed-off-by: Igor Mitsyanko <i.mitsyanko@samsung.com>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>