target-s390: Fail on unknown instructions

We were being a bit too nice and didn't give the guest an invalid instruction
interrupt.

While that works, it's not exactly the fastest thing to do, since now the
guest doesn't know that we're not really implementing that instruction, so it
continues doing it.

We run into this with the set_page_unstable hint instruction. So let's bail out
in these cases.

Signed-off-by: Alexander Graf <agraf@suse.de>
Signed-off-by: Aurelien Jarno <aurelien@aurel32.net>
