rbd: centralize rbd path concatenation

rbd: centralize rbd path concatenation

The <pool>/<image> paths are needed in quite a lot of places. Having one
single place where they are created helps to reduce duplicate code and
makes it easier to introduce new features.

The 'add_pool_to_disk' sub was already doing that but the name was not
really fitting. This commit renames it to the more general
'get_rbd_path' and changes the second parameter to the more widely used
$volume instead of $disk.

Furthermore, all occurences where "$pool/$volume" has been concatenated
have been replaced with a call to get_rbd_path.

Plus some minor code style cleanups for long function calls that were
touched.

Signed-off-by: Aaron Lauterer <a.lauterer@proxmox.com>