kvm: x86: merge kvm_arch_set_irq and kvm_arch_set_irq_inatomic

kvm_arch_set_irq is unused since commit b97e6de9.  Merge
its functionality with kvm_arch_set_irq_inatomic.

Reported-by: Jiang Biao <jiang.biao2@zte.com.cn>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Reviewed-by: David Hildenbrand <david@redhat.com>
Signed-off-by: Radim Krčmář <rkrcmar@redhat.com>