KVM: arm64: selftests: get-reg-list: Introduce vcpu configs

We already break register lists into sublists that get selected based
on vcpu config. However, since we only had two configs (vregs and sve),
we didn't structure the code very well to manage them. Restructure it
now to more cleanly handle register sublists that are dependent on the
vcpu config.

This patch has no intended functional change (except for the vcpu
config name now being prepended to all output).

Signed-off-by: Andrew Jones <drjones@redhat.com>
Reviewed-by: Ricardo Koller <ricarkol@google.com>
Signed-off-by: Marc Zyngier <maz@kernel.org>
Link: https://lore.kernel.org/r/20210531103344.29325-2-drjones@redhat.com