vmm: hypervisor: Add support for injecting NMI for MSHV guest

Currently, we only support injecting NMI for KVM guests but we can do
the same for MSHV guests as well to have feature parity.

Signed-off-by: Jinank Jain <jinankjain@microsoft.com>

hypervisor/src/cpu.rs

@@ -515,7 +515,5 @@ pub trait Vcpu: Send + Sync {
     ///
     /// Trigger NMI interrupt
     ///
-    fn nmi(&self) -> Result<()> {
-        unimplemented!()
-    }
+    fn nmi(&self) -> Result<()>;
 }