x86/kvm: Support Hyper-V reenlightenment

When running nested KVM on Hyper-V guests its required to update
masterclocks for all guests when L1 migrates to a host with different TSC
frequency.

Implement the procedure in the following way:
  - Pause all guests.
  - Tell the host (Hyper-V) to stop emulating TSC accesses.
  - Update the gtod copy, recompute clocks.
  - Unpause all guests.

This is somewhat similar to cpufreq but there are two important differences:
 - TSC emulation can only be disabled globally (on all CPUs)
 - The new TSC frequency is not known until emulation is turned off so
   there is no way to 'prepare' for the event upfront.

Signed-off-by: Vitaly Kuznetsov <[email protected]>
Signed-off-by: Thomas Gleixner <[email protected]>
Acked-by: Paolo Bonzini <[email protected]>
Cc: Stephen Hemminger <[email protected]>
Cc: [email protected]
Cc: Radim Krčmář <[email protected]>
Cc: Haiyang Zhang <[email protected]>
Cc: "Michael Kelley (EOSG)" <[email protected]>
Cc: Roman Kagan <[email protected]>
Cc: Andy Lutomirski <[email protected]>
Cc: [email protected]
Cc: "K. Y. Srinivasan" <[email protected]>
Cc: Cathy Avery <[email protected]>
Cc: Mohammed Gamal <[email protected]>
Link: https://lkml.kernel.org/r/[email protected]

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