s390: Fix cpu refactoring fallout.

s390: Fix cpu refactoring fallout.

Commit 259186a7 "cpu: Move halted and interrupt_request fields to CPUState"
seems to have missed one instance in target-s390x/kvm.c:

/home/cohuck/git/qemu/target-s390x/kvm.c: In function ‘kvm_arch_process_async_events’:
/home/cohuck/git/qemu/target-s390x/kvm.c:319: error: ‘CPUS390XState’ has no member named ‘halted’
/home/cohuck/git/qemu/target-s390x/kvm.c:320: warning: control reaches end of non-void function
make[1]: *** [target-s390x/kvm.o] Error 1

Let's just switch to cs->halted.

Signed-off-by: Cornelia Huck <cornelia.huck@de.ibm.com>
Acked-by: Alexander Graf <agraf@suse.de>
Acked-by: Andreas Färber <afaerber@suse.de>
Signed-off-by: Blue Swirl <blauwirbel@gmail.com>