ppc: Avoid decrementer related kvm exits

In __cpu_ppc_store_decr(), we set up a regular timer used to trigger
decrementer interrupts.  This is necessary to implement the decrementer
properly under TCG, but is unnecessary under KVM (true for both Book3S-PR
and Book3S-HV KVM variants), because the kernel handles generating and
delivering decrementer exceptions.

Under kvm, in fact, the timer causes expensive and unnecessary exits from
kvm to qemu.  This patch, therefore, disables setting the timer when kvm
is in use.

Signed-off-by: Anton Blanchard <anton@au1.ibm.com>
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
Signed-off-by: Alexander Graf <agraf@suse.de>