[BUG] Migration hv_time rollback

[Antoine Damhet]

Hi,

We are experiencing timestamp rollbacks during live-migration of
Windows 10 guests with the following qemu configuration (linux 5.4.46
and qemu master):
```
$ qemu-system-x86_64 -enable-kvm -cpu host,kvm=off,hv_time [...]
```

I have tracked the bug to the fact that `kvmclock` is not exposed and
disabled from qemu PoV but is in fact used by `hv-time` (in KVM).

I think we should enable the `kvmclock` (qemu device) if `hv-time` is
present and add Hyper-V support for the `kvmclock_current_nsec`
function.

I'm asking for advice because I am unsure this is the _right_ approach
and how to keep migration compatibility between qemu versions.

Thank you all,

-- 
Antoine 'xdbob' Damhet

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