KVM: PPC: Book3S HV nestedv2: Add support for reading VPA counters for pseries guests

PAPR hypervisor has introduced three new counters in the VPA area of
LPAR CPUs for KVM L2 guest (see [1] for terminology) observability - two
for context switches from host to guest and vice versa, and one counter
for getting the total time spent inside the KVM guest. Add a tracepoint
that enables reading the counters for use by ftrace/perf. Note that this
tracepoint is only available for nestedv2 API (i.e, KVM on PowerVM).

[1] Terminology:
a. L1 refers to the VM (LPAR) booted on top of PAPR hypervisor
b. L2 refers to the KVM guest booted on top of L1.

Reviewed-by: Nicholas Piggin <[email protected]>
Acked-by: Naveen N Rao <[email protected]>
Signed-off-by: Vaibhav Jain <[email protected]>
Signed-off-by: Gautam Menghani <[email protected]>
Signed-off-by: Michael Ellerman <[email protected]>
Link: https://msgid.link/[email protected]

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