kvm_arch_set_irq is unused since commit b97e6de9.  Merge
its functionality with kvm_arch_set_irq_inatomic.
Reported-by: NJiang Biao <jiang.biao2@zte.com.cn>
Signed-off-by: NPaolo Bonzini <pbonzini@redhat.com>
Reviewed-by: NDavid Hildenbrand <david@redhat.com>
Signed-off-by: NRadim Krčmář <rkrcmar@redhat.com>