spapr: Implement DR-indicator for physical DRCs only

According to PAPR, the DR-indicator should only be valid for physical DRCs,
not logical DRCs.  At the moment we implement it for all DRCs, so restrict
it to physical ones only.

We move the state to the physical DRC subclass, which means adding some
QOM boilerplate to handle the newly distinct type.

Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
Reviewed-by: Daniel Barboza <danielhb@linux.vnet.ibm.com>
Tested-by: Daniel Barboza <danielhb@linux.vnet.ibm.com>
