Use a common interface for find_free_diskname

Use a common interface for find_free_diskname

We can use 'list_images' to get the desired volume IDs in
'find_free_diskname' for most plugins. For the two LVM plugins, 'list_images'
potentially skips untagged volumes, so we keep the custom version. For the
RBD plugin, 'list_images' is much more costly than the custom version, so we
keep the custom version.

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