kunit: tool: drop last uses of collections.namedtuple

Since we formally require python3.7+ since commit df4b0807
("kunit: tool: Assert the version requirement"), we can just use
@dataclasses.dataclass instead.

In kunit_config.py, we used namedtuple to create a hashable type that
had `name` and `value` fields and had to subclass it to define a custom
`__str__()`.
@datalcass lets us just define one type instead.

In qemu_config.py, we use namedtuple to allow modules to define various
parameters. Using @dataclass, we can add type-annotations for all these
fields, making our code more typesafe and making it easier for users to
figure out how to define new configs.

Signed-off-by: Daniel Latypov <dlatypov@google.com>
Reviewed-by: David Gow <davidgow@google.com>
Reviewed-by: Brendan Higgins <brendanhiggins@google.com>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>