ioapic: support "info irq"

ioapic: support "info irq"

This include both userspace and in-kernel ioapic.  Note that the numbers
can be inaccurate for kvm-ioapic.  One reason is the same with
kvm-i8259, that when irqfd is used, irqs can be delivered all inside
kernel without our notice.  Meanwhile, kvm-ioapic is specially treated
when irq numbers <ISA_NUM_IRQS, those irqs will be delivered in kernel
too via kvm-i8259 (please refer to kvm_pc_gsi_handler).

Signed-off-by: Peter Xu <peterx@redhat.com>
Message-Id: <20171229073104.3810-5-peterx@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>