machine: replace query_hotpluggable_cpus() callback with has_hotpluggable_cpus flag

machine: replace query_hotpluggable_cpus() callback with has_hotpluggable_cpus flag

Generic helper machine_query_hotpluggable_cpus() replaced
target specific query_hotpluggable_cpus() callbacks so
there is no need in it anymore. However inon NULL callback
value is used to detect/report hotpluggable cpus support,
therefore it can be removed completely.
Replace it with MachineClass.has_hotpluggable_cpus boolean
which is sufficient for the task.

Suggested-by: David Gibson <david@gibson.dropbear.id.au>
Signed-off-by: Igor Mammedov <imammedo@redhat.com>
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>