libxl/arm: provide guests with random seed

libxl/arm: provide guests with random seed

Pass 128 bytes of random seed via FDT, so that guests' CRNGs are better seeded
early at boot. This is larger than ChaCha20 key size of 32, so each byte of
CRNG state will be mixed 4 times using this seed. There does not seem to be
advantage in larger seed though.

Depending on its configuration Linux can use the seed as device randomness
or to just quickly initialize CRNG.
In either case this will provide extra randomness to further harden CRNG.

Signed-off-by: Sergiy Kibrik <Sergiy_Kibrik@epam.com>
Reviewed-by: Julien Grall <julien@xen.org>
Reviewed-by: Michal Orzel <michal.orzel@arm.com>