fix #3610: properly build ZFS detail tree

fix #3610: properly build ZFS detail tree

Previously, top-level vdevs like log or special were wrongly added as
children of the previous outer vdev instead of the root.

Fix it by also showing the vdev with the same name as the pool and
start counting from level 1 (the pool itself serves as the root and
should be the only one with level 0). This results in the same kind
of structure as in PBS and (except for the root) zpool status itself.

Signed-off-by: Fabian Ebner <f.ebner@proxmox.com>