KVM: s390: clean up interrupt injection in sigp code

We have all the logic to inject interrupts available in
kvm_s390_inject_vcpu(), so let's use it instead of
injecting irqs manually to the list in sigp code.

SIGP stop is special because we have to check the
action_flags before injecting the interrupt. As
the action_flags are not available in kvm_s390_inject_vcpu()
we leave the code for the stop order code untouched for now.

Signed-off-by: Jens Freimann <jfrei@linux.vnet.ibm.com>
Reviewed-by: David Hildenbrand <dahi@linux.vnet.ibm.com>
Reviewed-by: Cornelia Huck <cornelia.huck@de.ibm.com>