plugin: change name, separator and error message for dir overrides

plugin: change name, separator and error message for dir overrides

Rename the "dirs" parameter to "content-dirs". Switch from a "vtype:/dir"
format to "vtype=/dir", and remove the misleading error message talking
about "absolute" paths. One might expect these to be absolute over the
whole system, while in reality they are relative to the mountpoint of
the storage.

Signed-off-by: Leo Nunner <l.nunner@proxmox.com>