KVM: PPC: Make PV mtmsr work with r30 and r31

So far we've been restricting ourselves to r0-r29 as registers an mtmsr
instruction could use. This was bad, as there are some code paths in
Linux actually using r30.

So let's instead handle all registers gracefully and get rid of that
stupid limitation

Signed-off-by: Alexander Graf <agraf@suse.de>