Node: add and export a 'used disk' function

This is similar to iMem.

Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: Balazs Lecz <leczb@google.com>

Ganeti/HTools/Node.hs

@@ -57,6 +57,7 @@ module Ganeti.HTools.Node
     , availMem
     , availCpu
     , iMem
+    , iDsk
     , conflictingPrimaries
     -- * Formatting
     , defaultFields
@@ -435,6 +436,10 @@ availDisk t =
        then 0
        else _f - _l
 
+-- | Computes the amount of used disk on a given node
+iDsk :: Node -> Int
+iDsk t = truncate (tDsk t) - fDsk t
+
 -- | Computes the amount of available memory on a given node
 availMem :: Node -> Int
 availMem t =