[Sheepdog] fix calculation of free disk spaces

[Piavlo]

Hi,

I've just build against the current next branch. The fd leak seems to be
solved, but  disk spaces are still not reported correctly

fire-srv3 ~ # shepherd info -t sheep
Id      Size    Used    Use%
 0      12G     0G        0%
 1      11G     0G        0%
 2      13G     0G        0%

Total   36G     0G        0%, total virtual VDI Size    5G
fire-srv3 ~ #        

ire-srv3 ~ # collie -h
Usage: collie [OPTION] [PATH]
Sheepdog Daemon, version de60024-d993db3
  -p, --port              specify the listen port number
  -f, --foreground        make the program run in the foreground
  -d, --debug             print debug messages
  -h, --help              display this help and exit
fire-srv3 ~ #