KVM: PPC: Remove comment saying SPRG1 is used for vcpu pointer

Scott Wood pointed out that We are no longer using SPRG1 for vcpu pointer,
but using SPRN_SPRG_THREAD <=> SPRG3 (thread->vcpu). So this comment
is not valid now.

Note: SPRN_SPRG3R is not supported (do not see any need as of now),
and if we want to support this in future then we have to shift to using
SPRG1 for VCPU pointer.

Signed-off-by: Bharat Bhushan <Bharat.Bhushan@freescale.com>
Signed-off-by: Alexander Graf <agraf@suse.de>