KVM: ppc: add lwzx/stwz emulation

Somehow these load/store instructions got missed before, but weren't used by
the guest so didn't break anything.

Signed-off-by: Hollis Blanchard <hollisb@us.ibm.com>
Signed-off-by: Christian Ehrhardt <ehrhardt@linux.vnet.ibm.com>
Signed-off-by: Avi Kivity <avi@qumranet.com>