KVM: PPC: Book3S HV: Fix instruction emulation

Commit 4a157d61

 ("KVM: PPC: Book3S HV: Fix endianness of
instruction obtained from HEIR register") had the side effect that
we no longer reset vcpu->arch.last_inst to -1 on guest exit in
the cases where the instruction is not fetched from the guest.
This means that if instruction emulation turns out to be required
in those cases, the host will emulate the wrong instruction, since
vcpu->arch.last_inst will contain the last instruction that was
emulated.

This fixes it by making sure that vcpu->arch.last_inst is reset
to -1 in those cases.

Signed-off-by: Paul Mackerras <paulus@samba.org>
Signed-off-by: Alexander Graf <agraf@suse.de>