check if base volumes are unused before deleting a template

check if base volumes are unused before deleting a template

we only checked if a vm had in use base disks when deleting them,
at which point we do not stop to delete the vm even when a
disk deletion fails, which means we could successfully delete the config
and all not used (base) disks of a template, resulting in left over vm disks

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