KVM: PPC: booke: add booke-level vcpu load/put

This gives us a place to put load/put actions that correspond to
code that is booke-specific but not specific to a particular core.

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