KVM: PPC: Book3S: Replace reset_msr mmu op with inject_interrupt arch op

reset_msr sets the MSR for interrupt injection, but it's cleaner and
more flexible to provide a single op to set both MSR and PC for the
interrupt.

Signed-off-by: Nicholas Piggin <npiggin@gmail.com>
Signed-off-by: Paul Mackerras <paulus@ozlabs.org>