hypervisor: provide a generic MpState structure

It is however only used for KVM right now because MSHV does not need it
yet.

Nonetheless a stub MSHV constructor should be there and get/set
functions should be implemented for MSHV.

Signed-off-by: Wei Liu <liuwe@microsoft.com>

hypervisor/src/cpu.rs

@@ -27,7 +27,6 @@ use crate::x86_64::{
 use crate::CpuState;
 #[cfg(target_arch = "aarch64")]
 use crate::DeviceAttr;
-#[cfg(feature = "kvm")]
 use crate::MpState;
 use thiserror::Error;
 #[cfg(all(feature = "kvm", target_arch = "x86_64"))]
@@ -351,12 +350,10 @@ pub trait Vcpu: Send + Sync {
     /// Setup the model-specific registers (MSR) for this vCPU.
     ///
     fn set_msrs(&self, msrs: &MsrEntries) -> Result<usize>;
-    #[cfg(feature = "kvm")]
     ///
     /// Returns the vcpu's current "multiprocessing state".
     ///
     fn get_mp_state(&self) -> Result<MpState>;
-    #[cfg(feature = "kvm")]
     ///
     /// Sets the vcpu's current "multiprocessing state".
     ///