Test non-default -smp core and thread counts and a non-default CPU model
on all PC machines except for isapc. Note that not all historic versions
actually supported this particular configuration, ignored for simplicity.

For machines pc-*-1.5+ test QMP cpu-add with monotonically increasing ID,
and test for graceful failure otherwise.

Cc: Eduardo Habkost <ehabkost@redhat.com>
Signed-off-by: Andreas Färber <afaerber@suse.de>