KVM: arm64: Reduce overhead of trapped timer sysreg accesses

Each read/write to a trapped timer system register results
in a whole kvm_timer_vcpu_put/load() cycle which affects all
of the timers, and a bit more.

There is no need for such a thing, and we can limit the impact
to the timer being affected, and only this one.

This drastically simplifies the emulated case, and limits the
damage for trapped accesses. This also brings some performance
back for NV.

Whilst we're at it, fix a comment that didn't quite capture why
we always set CNTVOFF_EL2 to 0 when disabling the virtual timer.

Signed-off-by: Marc Zyngier <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Oliver Upton <[email protected]>

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