vm-device: reevaluate #[allow] attributes

Drop stale #[allow]s whose lints no longer fire and convert the
still-needed ones to #[expect] so they warn if the lints stop firing.

Part of #8326.

Signed-off-by: Tushar Khatri <hello@tusharkhatri.in>

vm-device/src/interrupt/mod.rs

@@ -164,7 +164,6 @@ pub trait InterruptSourceGroup: Send + Sync {
     /// An interrupt notifier allows for external components and processes
     /// to inject interrupts into a guest, by writing to the [`EventFd`] returned
     /// by this method.
-    #[allow(unused_variables)]
     fn notifier(&self, index: InterruptIndex) -> Option<EventFd>;
 
     /// Update the interrupt source group configuration.