KVM: s390: floating irqs: fix user triggerable endless loop

If a vm with no VCPUs is created, the injection of a floating irq
leads to an endless loop in the kernel.

Let's skip the search for a destination VCPU for a floating irq if no
VCPUs were created.

Reviewed-by: Dominik Dingel <dingel@linux.vnet.ibm.com>
Reviewed-by: Cornelia Huck <cornelia.huck@de.ibm.com>
Signed-off-by: David Hildenbrand <dahi@linux.vnet.ibm.com>
Cc: stable@vger.kernel.org # v3.15+
Signed-off-by: Christian Borntraeger <borntraeger@de.ibm.com>