KVM: PPC: Preload FPU when possible

There are some situations when we're pretty sure the guest will use the
FPU soon. So we can save the churn of going into the guest, finding out
it does want to use the FPU and going out again.

This patch adds preloading of the FPU when it's reasonable.

Signed-off-by: Alexander Graf <agraf@suse.de>
Signed-off-by: Avi Kivity <avi@redhat.com>