x86/hvm: replace architecture TSC scaling by a common function

x86/hvm: replace architecture TSC scaling by a common function

This patch implements a common function hvm_scale_tsc() to scale TSC by
using TSC scaling information collected by architecture code.

Signed-off-by: Haozhong Zhang <haozhong.zhang@intel.com>
Acked-by: Boris Ostrovsky <boris.ostrovsky@oracle.com> for SVM bits
Reviewed-by: Jan Beulich <jbeulich@suse.com>