hypervisor: Set destination vCPU TSC frequency to source

Include the TSC frequency as part of the KVM state so that it will be
restored at the destination.

This ensures migration works correctly between hosts that have a
different TSC frequency if the guest is running with TSC as the source
of timekeeping.

Fixes: #5786

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

hypervisor/src/kvm/x86_64/mod.rs

@@ -67,6 +67,7 @@ pub struct VcpuKvmState {
     pub xsave: Xsave,
     pub xcrs: ExtendedControlRegisters,
     pub mp_state: MpState,
+    pub tsc_khz: Option<u32>,
 }
 
 impl From<StandardRegisters> for kvm_regs {