lguest: struct device - replace bus_id with dev_name()

bus_id is gradually being removed, so use dev_name() instead.

Signed-off-by: Mark McLoughlin <markmc@redhat.com>
Cc: Kay Sievers <kay.sievers@vrfy.org>
Cc: Greg Kroah-Hartman <gregkh@suse.de>
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>