selftests: Move device validation in nettest

Later patch adds support for switching network namespaces before
running client, server or both. Device validations need to be
done after the network namespace switch, so add a helper to do it
and invoke in server and client code versus inline with argument
parsing. Move related argument checks as well.

Signed-off-by: David Ahern <dsahern@gmail.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>