xenserver: Generate system UUID for non-XenServer systems.

xenserver: Generate system UUID for non-XenServer systems.

Currently the scripts in xensever/ are intended specifically for XenServer,
but supporting other distros seems like a worthy goal, so this is a step
in the right direction.

It would be good to be able to determine a suitable system-type and
system-version, but those are less important than system-id.

Signed-off-by: Patrick Mullaney <pm.mullaney@gmail.com>
[changed not to set xs-system-uuid on non-XenServer systems]
[whitespace changes]
[changed to handle missing uuidgen, suggested by Justin Pettit]
Signed-off-by: Ben Pfaff <blp@nicira.com>