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>
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