fix #2467: avoid duplicate volumes & tag with correct content type

fix #2467: avoid duplicate volumes & tag with correct content type

The bugfix for #2317 introduced a kind of odd API behavior, where
each volume was returned twice from our API if a storage has both
'rootdir' & 'images' content types enabled. To give the content type
of the volume an actual meaning, it is now inferred from the
associated guest, if there's no guest or we don't have an owner for
that volume we default to 'images'.

At the volume level, there is no option to list volumes based on
content types, since the volumes do not know what type they are
actually used for.

Signed-off-by: Tim Marx <t.marx@proxmox.com>