Read system UUID from DMI and merge hostnqn generation functions

Read system UUID from DMI and merge hostnqn generation functions

Both fabrics.c and the 'gen-hostnqn' command contain functions
to generate a default host NQN. Needless to say, both are different.
_And_ we have the script 'nvme-gen-hostnqn' to read the host nqn
from the system UUID, implement yet another way.

This patch merges all of these functions, and implements a hierarchy
in generating host NQNs:
- Try reading system UUID from /sys/firmware/dmi/entries
- Try reading systemd-generated UUID
- Generate a random UUID

Signed-off-by: Hannes Reinecke <hare@suse.de>