low-level: install: check for already-existing `rpool` on install

low-level: install: check for already-existing `rpool` on install

.. much in the same manner as the detection for LVM works.

zpools can only be renamed by importing them with a new name, so
unfortunately the import-export dance is needed.

Signed-off-by: Christoph Heiss <c.heiss@proxmox.com>
Reviewed-by: Aaron Lauterer <a.lauterer@proxmox.com>
Tested-by: Aaron Lauterer <a.lauterer@proxmox.com>
Signed-off-by: Thomas Lamprecht <t.lamprecht@proxmox.com>