hmp: Add 'info hotpluggable-cpus' HMP command

This is the HMP equivalent for QMP query-hotpluggable-cpus.

Signed-off-by: Bharata B Rao <bharata@linux.vnet.ibm.com>
Reviewed-by: David Gibson <david@gibson.dropbear.id.au>
[dwg: Fixed problem with printf formats on 32-bit host]
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>