KVM: s390: ignore IBC if zero

Looks like we forgot about the special IBC value of 0 meaning "no IBC".
Let's fix that, otherwise it gets rounded up and suddenly an IBC is active
with the lowest possible machine.

Signed-off-by: David Hildenbrand <dahi@linux.vnet.ibm.com>
Reviewed-by: Cornelia Huck <cornelia.huck@de.ibm.com>
Fixes: commit 053dd2308d81 ("KVM: s390: force ibc into valid range")
Signed-off-by: Christian Borntraeger <borntraeger@de.ibm.com>