interrupt: Add a notifier method to the InterruptController

Both GIC and IOAPIC must implement a new method notifier() in order to
provide the caller with an EventFd corresponding to the IRQ it refers
to.

This is needed in anticipation for supporting INTx with VFIO PCI
devices.

Signed-off-by: Sebastien Boeuf <sebastien.boeuf@intel.com>

devices/src/interrupt_controller.rs

@@ -4,6 +4,7 @@
 
 use std::io;
 use std::result;
+use vmm_sys_util::eventfd::EventFd;
 
 #[derive(Debug)]
 pub enum Error {
@@ -56,4 +57,5 @@ pub trait InterruptController: Send {
     fn enable(&self) -> Result<()>;
     #[cfg(target_arch = "x86_64")]
     fn end_of_interrupt(&mut self, vec: u8);
+    fn notifier(&self, irq: usize) -> Option<EventFd>;
 }