libxl: add libxl__random_bytes() which fills a buffer with random bytes

libxl: add libxl__random_bytes() which fills a buffer with random bytes

The random bytes are obtained from /dev/urandom and are suitable for
almost all uses (except for generating long-lived secure keys).

Documentation suggests that /dev/urandom is widely available on Unix-like
systems (such FreeBSD and NetBSD).

A public libxl_random_bytes() (or similar) could be trivially added,
if this required in the future.

Signed-off-by: David Vrabel <david.vrabel@citrix.com>
Acked-by: Ian Campbell <ian.campbell@citrix.com>