KVM: arm64: PMU: Make kvm_pmc the main data structure

The PMU code has historically been torn between referencing a counter
as a pair vcpu+index or as the PMC pointer.

Given that it is pretty easy to go from one representation to
the other, standardise on the latter which, IMHO, makes the
code slightly more readable. YMMV.
Signed-off-by: Marc Zyngier <maz@kernel.org>
Link: https://lore.kernel.org/r/20221113163832.3154370-17-maz@kernel.org