[pve-devel] qemu-server: replace file_size_info by volume_size_info

[Alexandre Derumier]

Currently we only find a volume size by using qemu-img info (in file_size_info).

This doesn't works with "virtual" storage like sheepdog,rbd,iscsidirect,nexenta.

This also doesn't work with classic iscsi direct plugin.

So, we need to define a method for each plugin, and avoid to call path and file_size_info when it's not necessary.