pseries: Restore support for total vcpus not a multiple of threads-per-core for old machine types

As of pseries-2.7 and later, we require the total number of guest vcpus to
be a multiple of the threads-per-core.  pseries-2.6 and earlier machine
types, however, are supposed to allow this for the sake of migration from
old qemu versions which allowed this.

Unfortunately, 8149e299 "pseries: Enforce homogeneous threads-per-core"
broke this by not considering the old machine type case.  This fixes it by
only applying the check when the machine type supports hotpluggable cpus.
By not-entirely-coincidence, that corresponds to the same time when we
started enforcing total threads being a multiple of threads-per-core.

Fixes: 8149e299

Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
Reviewed-by: Laurent Vivier <lvivier@redhat.com>
Reviewed-by: Greg Kurz <groug@kaod.org>
Tested-by: Greg Kurz <groug@kaod.org>