Include DRBD overhead in sda/sdb size

For Ganeti 1.2 which doesn't have the ‘disk_usage’ instance query field,
we need to manually include the DRBD overhead (per disk). This patch
modifies the RAPI collection to do this, but loading from disk does not
as it's unknown if the query came from hscan or RAPI 1.2 or RAPI 2.0...

Ganeti/HTools/Rapi.hs

@@ -19,6 +19,11 @@ import Text.Printf (printf)
 import Ganeti.HTools.Utils ()
 
 
+-- Some constants
+
+-- | The fixed drbd overhead per disk (only used with 1.2's sdx_size)
+drbdOverhead = 128
+
 {-- Our cheap monad-like stuff.
 
 Thi is needed since Either e a is already a monad instance somewhere
@@ -141,9 +146,10 @@ parseInstance :: JSObject JSValue -> Either String String
 parseInstance a =
     let name = getStringElement "name" a
         disk = case getIntElement "disk_usage" a of
-                 Left _ -> apply2 (+)
-                           (getIntElement "sda_size" a)
-                           (getIntElement "sdb_size" a)
+                 Left _ -> let log_sz = apply2 (+)
+                                        (getIntElement "sda_size" a)
+                                        (getIntElement "sdb_size" a)
+                           in apply2 (+) log_sz (Right $ drbdOverhead * 2)
                  Right x -> Right x
         bep = fromObj "beparams" a
         pnode = getStringElement "pnode" a