KVM: PPC: Book3S HV: Take out virtual core piggybacking code

This takes out the code that arranges to run two (or more) virtual
cores on a single subcore when possible, that is, when both vcores
are from the same VM, the VM is configured with one CPU thread per
virtual core, and all the per-subcore registers have the same value
in each vcore.  Since the VTB (virtual timebase) is a per-subcore
register, and will almost always differ between vcores, this code
is disabled on POWER8 machines, meaning that it is only usable on
POWER7 machines (which don't have VTB).  Given the tiny number of
POWER7 machines which have firmware that allows them to run HV KVM,
the benefit of simplifying the code outweighs the loss of this
feature on POWER7 machines.

Tested-by: Thomas Huth <thuth@redhat.com>
Signed-off-by: Paul Mackerras <paulus@ozlabs.org>