Virt: Have a way to change the random seed

[sosthene-nitrokey]

Right now the random seed is always set to [42u8;32], which is fine for tests. It might be worth have a way to change it, for example in fuzzing it would be desirable to not generate always the same keys, and extract the seed from the fuzz input.

[robin-nitrokey]

Probably, the default should be a random seed, providing the option to override it with a static seed for reproducibility tests or a seed from a different source of randomness. Unfortunately I don’t know what regression is meant by this comment:

trussed/src/virt.rs

Lines 38 to 39 in baab96d

	// causing a regression again
	// let rng = chacha20::ChaCha8Rng::from_rng(rand_core::OsRng).unwrap();

Suggestion:

1. Introduce struct Options { seed: Option<...>, }.
2. Add with_platform_options and with_client_options and use Options::default() in the existing methods.
3. If no seed is set in the options, generate it randomly. Otherwise, use the specified seed.