virtio-devices: Simplify interrupt handling

Previously the interrupt was created in VirtioPciDevice, moved via the
Option::take() to the VirtioPciDeviceActivator and then moved to the
VirtioDevice upon activation. On reset it would be moved back ready for
reactivation.

Since this already an Arc type remove the wrapping Option and instead
refcount it such that the VirtioPciDevice can continue to hold onto it
for later activations.

This significantly simplifies the reset() logic as there is no need to
hand back the interrupt.

A few devices used whether the interrupt was Some to make triggering an
interrupt a no-op. However the MSI-X interrupt routing already drops the
interrupt if the driver hasn't yet configured the vector so it is safe
to trigger the interrupt before device activation (e.g. balloon resize
request before driver loaded).

VirtioCommon still retains an Option<..> for the interrupt as the
interrupt is not known until activation time (after this has been
created). A helper VirtioCommon::trigger_interrupt() has been added to
handle this.

Signed-off-by: Rob Bradford <rbradford@meta.com>

virtio-devices/src/vdpa.rs

@@ -443,14 +443,13 @@ impl VirtioDevice for Vdpa {
         self.activate_vdpa(&mem.memory(), virtio_interrupt.as_ref(), &queues)
             .map_err(ActivateError::ActivateVdpa)?;
 
-        // Store the virtio interrupt handler as we need to return it on reset
         self.common.interrupt_cb = Some(virtio_interrupt);
 
         event!("vdpa", "activated", "id", &self.id);
         Ok(())
     }
 
-    fn reset(&mut self) -> Option<Arc<dyn VirtioInterrupt>> {
+    fn reset(&mut self) {
         // Backend reset failures are logged but don't skip local cleanup:
         // reset must converge to fresh state regardless of backend state.
         if let Err(e) = self.reset_vdpa() {
@@ -459,8 +458,8 @@ impl VirtioDevice for Vdpa {
 
         event!("vdpa", "reset", "id", &self.id);
 
-        // Return the virtio interrupt handler
-        self.common.interrupt_cb.take()
+        // Drop the interrupt callback clone
+        self.common.interrupt_cb = None;
     }
 
     fn set_access_platform(&mut self, access_platform: Arc<dyn AccessPlatform>) {