KVM: PPC: Don't reload FPU with invalid values

When the guest activates the FPU, we load it up. That's fine when
it wasn't activated before on the host, but if it was we end up
reloading FPU values from last time the FPU was deactivated on the
host without writing the proper values back to the vcpu struct.

This patch checks if the FPU is enabled already and if so just doesn't
bother activating it, making FPU operations survive guest context switches.

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