tools/xenstored: Propagate correctly the error message from lu_start()

tools/xenstored: Propagate correctly the error message from lu_start()

lu_start() will only set errno when it returns NULL. For all the
other cases, the value is unknown.

This means that when lu_start() returns an error message, it may not
be propagated to the client.

The check that errno is a non-zero value is now dropped and instead
the value is returned when no error message is provided. This
relies on errno to always be set when ret == NULL.

Fixes: af216a99fb ("tools/xenstore: add the basic framework for doing the live update")
Signed-off-by: Julien Grall <jgrall@amazon.com>
Reviewed-by: Juergen Gross <jgross@suse.com>