x86/hvm: setup TSC scaling ratio

x86/hvm: setup TSC scaling ratio

This patch adds a field tsc_scaling_ratio in struct hvm_domain to record
the per-domain TSC scaling ratio, and sets it in tsc_set_info().

Before setting the per-domain TSC scaling ratio, we check its validity
in tsc_set_info(). If an invalid ratio is given, we will leave the
default value in tsc_scaling_ratio (i.e. ratio = 1) and setup guest TSC
as if no TSC scaling is used:
* For TSC_MODE_DEFAULT,
  - if a user-specified TSC frequency is given, we will set the guest
    TSC frequency to it; otherwise, we set it to the host TSC frequency.
  - if guest TSC frequency does not equal to host TSC frequency, we will
    emulate guest TSC (i.e. d->arch.vtsc is set to 1). In both cases,
    guest TSC runs in the guest TSC frequency.
* For TSC_MODE_PVRDTSCP,
  - we set the guest TSC frequency to the host TSC frequency.
  - guest rdtsc is executed natively in the host TSC frequency as
    before.
  - if rdtscp is not available to guest, it will be emulated; otherwise,
    it will be executed natively. In both cases, guest rdtscp gets TSC
    in the host TSC frequency as before.

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