s390x/kvm: always ignore empty vcpu interrupt state

kvm_s390_vcpu_interrupt_pre_save() and
kvm_s390_vcpu_interrupt_post_load() are essentially no-ops on hosts
without KVM_CAP_S390_IRQ_STATE. Move the capability check after the
check for saved IRQ state in kvm_s390_vcpu_interrupt_post_load() so that
migration between hosts without KVM_CAP_S390_IRQ_STATE (including save /
restore on the same host) continues to work.

Fixes: 3cda44f7bae5 ("s390x/kvm: migrate vcpu interrupt state")
Signed-off-by: Sascha Silbe <silbe@linux.vnet.ibm.com>
Reviewed-by: David Hildenbrand <dahi@linux.vnet.ibm.com>
Reviewed-by: Jens Freimann <jfrei@linux.vnet.ibm.com>
Signed-off-by: Christian Borntraeger <borntraeger@de.ibm.com>
