kvm: x86: Update tsc multiplier on change.

kvm: x86: Update tsc multiplier on change.

vmx.c writes the TSC_MULTIPLIER field in vmx_vcpu_load, but only when a
vcpu has migrated physical cpus. Record the last value written and
update in vmx_vcpu_load on any change, otherwise a cpu migration must
occur for TSC frequency scaling to take effect.

Cc: stable@vger.kernel.org
Fixes: c877c72a0ac54c2f0187a2a67f5e6cd2be77e423
Signed-off-by: Owen Hofmann <osh@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>