Enforce InstanceIdentifier creation

We currently allow an InstanceIdentifier to be created for any
DataObject, which is wrong, as that includes raw RpcInputs,
Notifications and the like.

Update create() method to require proper relationship with DataRoot
instances. Alternatives can be worked through instantiating a proper
builder.

Also enforce individual items to always have the proper type.

JIRA: MDSAL-370
Change-Id: I38e83760c0ec29613a9ded24c0783bef940c9b7e
Signed-off-by: Robert Varga <robert.varga@pantheon.tech>