KVM: s390: count vcpu wakeups in stat.halt_wakeup

This patch introduces the halt_wakeup counter used by common code and uses it to
count vcpu wakeups done in s390 arch specific code.

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>