KVM: s390: sigp: dispatch orders with one target in a separate function

All sigp orders except SIGP SET ARCHITECTURE target exactly one vcpu.

Let's move the dispatch code for these orders into a separate function to
prepare for cleaner target availability checks.

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