bootdevice: move code about bootorder from vl.c to bootdevice.c

bootdevice: move code about bootorder from vl.c to bootdevice.c

First, we can downsize vl.c, make it simpler by
little and little. Second, I can maintain those code
and make some improvement.

Cc: Jan Kiszka <jan.kiszka@siemens.com>
Signed-off-by: Gonglei <arei.gonglei@huawei.com>
Reviewed-by: Markus Armbruster <armbru@redhat.com>