KVM: APIC: add helper func to remove duplicate code in kvm_pv_send_ipi

There are some duplicate code in kvm_pv_send_ipi when deal with ipi
bitmap. Add helper func to remove it, and eliminate odd out label,
get rid of unnecessary kvm_lapic_irq field init and so on.

Signed-off-by: Miaohe Lin <linmiaohe@huawei.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>