vmm: reevaluate #[allow] attributes

Remove stale #[allow]s whose lints no longer fire, convert the
unconditionally-firing ones to #[expect], and keep the
arch/feature-conditional ones as #[allow]. Verified across kvm/mshv,
x86_64/aarch64, and --all-features.

Part of #8326.

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

vmm/src/cpu.rs

@@ -842,8 +842,8 @@ impl VcpuState {
 }
 
 impl CpuManager {
-    #[allow(unused_variables)]
-    #[allow(clippy::too_many_arguments)]
+    #[expect(unused_variables)]
+    #[expect(clippy::too_many_arguments)]
     pub fn new(
         config: &CpusConfig,
         vm: Arc<dyn hypervisor::Vm>,