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: ff2c3a18
Signed-off-by: Owen Hofmann <osh@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>