loader: rewrite zfs vdev initialization

In some cases the pool discovery will get stuck in infinite loop while setting
up the vdev children.

To fix, we split the vdev setup into two parts, first we create vdevs based on
configuration we do get from pool label, then, we process pool config from MOS
and update the pool config if needed.

Testing done: confirm previously hung loader is not hung any more.

MFC after: 1 week