spapr: Increase hotpluggable memory slots to 256

spapr: Increase hotpluggable memory slots to 256

KVM now supports 512 memslots on PowerPC (earlier it was 32). Allow half
of it (256) to be used as hotpluggable memory slots.

Instead of hard coding the max value, use the KVM supplied value if KVM
is enabled. Otherwise resort to the default value of 32.

Signed-off-by: Bharata B Rao <bharata@linux.vnet.ibm.com>
Reviewed-by: Thomas Huth <thuth@redhat.com>
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>