mllib: Add a utility function for safely reading from /dev/urandom.

OCaml's buffered 'in_channel' has a 64k buffer, so using it to read a
few bytes from /dev/urandom removes a lot of the system's entropy (for
example /proc/sys/kernel/random/entropy_avail goes from ~3000 to 128).

This patch was originally by Edwin Török for builder.ml.  I
generalized it because there are two other places where we did
over-sized reads from /dev/urandom.
Avoid modulo bias in random password generation

Char.code (input_char chan) mod nr_chars has modulo bias because
the original interval is not a multiple of the destination interval,
i.e. 256 mod nr_chars != 0.

One way to fix this is to keep generating random numbers until they fall outside
the interval where modulo bias occurs, that is accept only c=[256 % nr_chars, 256).
That interval maps back to [0, nr_chars), and has a length of
(256 - 256 % nr_chars), which is a multiple of nr_chars.

RWMJ:
 - Modify the code so it goes into a utility library.
 - Use the same code across virt-builder and virt-sysprep.