kvm: ppc: booke: Use the shared struct helpers for SPRN_SPRG0-7

Use kvmppc_set_sprg[0-7]() and kvmppc_get_sprg[0-7]() helper
functions

Signed-off-by: Bharat Bhushan <Bharat.Bhushan@freescale.com>
Signed-off-by: Alexander Graf <agraf@suse.de>