fix #2474: always show iscsi content

fix #2474: always show iscsi content

Instead of relying on list_volumes of Plugin.pm (which filters by
the content types set in the config), use our own to always
show the luns of an iscsi.

This makes sense here, since we need it to show the luns when using
it as base storage for LVM (where we have content type 'none' set).

It does not interfere with the rest of the GUI, since on e.g. disk
creation, we already filter the storages in the dropdown by content
type, iow. an iscsi storage used this way still does not show up
when trying to create a disk.

This also shows the luns now in the 'Content' tab, but this is also
OK, since the user cannot actually do anything there with the luns.
(Besides looking at them)

Signed-off-by: Dominik Csapak <d.csapak@proxmox.com>