entropy: feed qubes random-seed at startup

[hannesm]

see mirage/mirage-qubes#34 for a discussion, basically there is a value for the key /qubes-random-seed in QubesDB that is provided by the host (once at startup!) to the VM to have some random bytes. this means that the rng is only available once QubesDB is up and running (does QubesDB initialization require randomness?), or the QubesDB connect code should enrich the (already seeded) RNG with some more entropy.

it would need to be integrated into entropy/, leading to an enriched API for target-dependent entropy harvesting (/bootstrapping).