hw/ppc/spapr_drc.c: change spapr_drc_needed to use drc->dev

This patch makes a small fix in 'spapr_drc_needed' to change how we detect
if a DRC has a device attached. Previously it used dr_entity_sense for this,
which  works for physical DRCs.

However, for logical DRCs, it didn't cover the case where a logical DRC has
a drc->dev but the state is LOGICAL_UNUSABLE (e.g. a hotplugged CPU before
CAS). In this case, the dr_entity_sense of this DRC returns UNUSABLE and the
code was considering that there were no dev attached, making spapr_drc_needed
return 'false' when in fact we would like to migrate the DRC.

Changing it to check for drc->dev instead works for all DRC types.

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