migrate: enforce that image content type is available

migrate: enforce that image content type is available

and use it for the vdisk_list call too. This avoids scanning (and picking up
volumes from!) storages that are not even configured to hold images.

Previously, the content type was only enforced when a storage map was present.

Also serves a bit as a preparation to enforce content type on guest startup,
because now migration failure happens early and not only when trying to start
the guest on the remote node.

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