KVM: arm64: selftests: get-reg-list: Provide config selection option

Add a new command line option that allows the user to select a specific
configuration, e.g. --config=sve will give the sve config. Also provide
help text and the --help/-h options.

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/20210531103344.29325-4-drjones@redhat.com