KVM: PPC: bookehv: Fix r8/r13 storing in level exception handler

Guest r8 register is held in the scratch register and stored correctly,
so remove the instruction that clobbers it. Guest r13 was missing from vcpu,
store it there.

Signed-off-by: Mihai Caraman <mihai.caraman@freescale.com>
Signed-off-by: Alexander Graf <agraf@suse.de>