pve6to7: more fine-grained detection of misconfigured guest volumes

pve6to7: more fine-grained detection of misconfigured guest volumes

If neither 'rootdir' nor 'images' are configured on a storage, but
there are guest images, just log the number of volumes found. If they
are relevant for migration, the check for unreferenced volumes will
catch them later.

Also detect content type mismatch for all volumes of existing guests,
which also covers the case of a VM image on a storage with only
'rootdir' and vice versa. To catch all such unreferenced volumes too,
it is necessary to scan all storages that do not have both content
types configured.

Change the message from 'will not work' to 'might not work'. If a
volume only referenced by a snapshot is misconfigured, it doesn't mean
that the guest doesn't work at all. Or it might be an ISO on a
misconfigured storage.

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