ui: add wizard to allow importing from ESXi attached as storage

ui: add wizard to allow importing from ESXi attached as storage

Add a new 'import' panel for storages supporting the 'import'
content type that shows a list of configs to import.

When opening the wizard, we query the meta info from the new
import-metadata API endpoint, and pre-fill the fields and shows
potential warnings or things to watch out for, returned by the API.

For disks and networks we allow to select which one to use and which
storage/bridge to import to.

Additionally, users can opt-in to a live-import, where the VM is
immediately started and storage requests are fetched from the target
on demand while importing the rest in the background.

Signed-off-by: Dominik Csapak <d.csapak@proxmox.com>
 [ TL: some fixes, clean-ups and commit message rewording ]
Signed-off-by: Thomas Lamprecht <t.lamprecht@proxmox.com>