KVM: x86: Use kernel timekeeping snapshots for getting kvmclock time since boot

Replace the KVM-private vgettsc()+do_kvmclock_base() timekeeping
reimplementation with calls to the recently crafted, generic
ktime_get_snapshot_id() interface.  This is the first step towards dropping
KVM's homebrewed implementation entirely (do_monotonic() and do_realtime()
will be converted in the near future).

As with KVM's implementation, the snapshot provides both the system time
and the raw_cycles (TSC), atomically paired using a sequence counter.  The
equivalents to vgettsc()'s TSC and HVCLOCK modes respectively are if the
clocksource itself is TSC (cs_id == CSID_X86_TSC) and if the underlying
hardware clocksource is TSC (hw_csid == CSID_X86_TSC).  In the Hyper-V
case, i.e. hw_csid == CSID_X86_TSC, if the clocksource couldn't provide a
raw hardware counter value, treat the clock not being based on TSC, which
which is equivalent to vgettsc() returning VDSO_CLOCKMODE_NONE.

Unlike KVM's current implementation, don't include offs_boot in the
atomically-acquired tuple as there's simply no need to do so: the time
since boot only changes at boot (duh) and at suspend/resume boundaries.
Unless processes aren't being frozen/thawed before/after suspend/resume,
which would completely break suspend/resume, TK_OFFS_BOOT can't change
while kvm_get_time_and_clockread() is running.  And if KVM does somehow try
to take a snapshot during suspend, timekeeping core will WARN and refuse to
provide the snapshot.

This is a step towards eliminating the pvclock_gtod_data private copy
of timekeeping state and the associated notifier callback.

Signed-off-by: David Woodhouse <dwmw@amazon.co.uk>
[sean: separate from other conversions, massage changelog accordingly]
Link: https://patch.msgid.link/20260826213303.914988-20-seanjc@google.com
Signed-off-by: Sean Christopherson <seanjc@google.com>

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