KVM: PPC: Make build work without CONFIG_VSX/ALTIVEC

The FPU/Altivec/VSX enablement also brought access to some structure
elements that are only defined when the respective config options
are enabled.

Unfortuately I forgot to check for the config options at some places,
so let's do that now.

Unbreaks the build when CONFIG_VSX is not set.

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