qemu: Make monitor aware of CPU clusters

qemu: Make monitor aware of CPU clusters

This makes it so libvirt can obtain accurate information about
guest CPUs from QEMU, and should make it possible to correctly
perform operations such as CPU hotplug.

Of course this is mostly moot at the moment: only aarch64 can use
CPU clusters, and CPU hotplug is not yet implemented on that
architecture.

Signed-off-by: Andrea Bolognani <abologna@redhat.com>
Reviewed-by: Peter Krempa <pkrempa@redhat.com>