x86: Add IA32_TSC_AUX MSR and use it

Clean up write_tsc() and write_tscp_aux() by replacing
hardcoded values.

No change in functionality.

Signed-off-by: Sheng Yang <sheng@linux.intel.com>
Cc: Avi Kivity <avi@redhat.com>
Cc: Marcelo Tosatti <mtosatti@redhat.com>
LKML-Reference: <1260942485-19156-4-git-send-email-sheng@linux.intel.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>