xl: randomly generate UUIDs

xl: randomly generate UUIDs

This patch converts xl to randomly generate UUID's rather than using a
dodgy time-seeded PRNG. I have ignored various suggestions so far on
auto-generation of MAC addresses and left it as a topic for a future
patch to solve. In other words the behaviour stays the same it's just
using a true random source. This patch also implements the "uuid" config
file parameter in xl.

Signed-off-by: Gianni Tedesco <gianni.tedesco@citrix.com>
Acked-By: Christoph Egger <Christoph.Egger@amd.com>
Signed-off-by: Ian Jackson <ian.jackson@eu.citrix.com>