Currently we have no way to define a stable host-id but always use the one
which is randomly generated when we add the host or use the default host.

Provide a "hostid=%s" for user-space to pass in a persistent host-id which
overrides the randomly generated one.
Signed-off-by: Johannes Thumshirn <jthumshirn@suse.de>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Sagi Grimberg <sagi@grimberg.me>
Signed-off-by: Keith Busch <keith.busch@intel.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>