KVM: PPC: Add support for FPU/Altivec/VSX

When our guest starts using either the FPU, Altivec or VSX we need to make
sure Linux knows about it and sneak into its process switching code
accordingly.

This patch makes accesses to the above parts of the system work inside the
VM.

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