KVM: arm64: selftests: get-reg-list: Prepare to run multiple configs at once

We don't want to have to create a new binary for each vcpu config, so
prepare to run the test for multiple vcpu configs in a single binary.
We do this by factoring out the test from main() and then looping over
configs. When given '--list' we still never print more than a single
reg-list for a single vcpu config though, because it would be confusing
otherwise.

No functional change intended.

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/20210531103344.29325-3-drjones@redhat.com