KVM: s390: prevent sleep duration underflows in handle_wait()

We sometimes get an underflow for the sleep duration, which most
likely won't result in the short sleep time we wanted.

So let's check for sleep duration underflows and directly continue
to run the guest if we get one.

Acked-by: Christian Borntraeger <borntraeger@de.ibm.com>
Acked-by: Cornelia Huck <cornelia.huck@de.ibm.com>
Signed-off-by: David Hildenbrand <dahi@linux.vnet.ibm.com>
Signed-off-by: Christian Borntraeger <borntraeger@de.ibm.com>