df: Rewrite virt-df in C.

I have diffed the output from the original virt-df with this
new version, and they agree very closely.  Some differences:

 - Old virt-df have a divide-by-zero error in cases where the
   number of used inodes was 0.  New virt-df fixes this.

 - New virt-df uses gnulib human_readable library which displays
   numbers to 3 significant figures for -h output (old version
   used an ad hoc function).

1 files changed, 3 insertions, 0 deletions

diff --git a/HACKING b/HACKING
index 05fb97ba..3ea0e4ae 100644
--- a/HACKING
+++ b/HACKING
@@ -84,6 +84,9 @@ csharp/
 daemon/
         The daemon that runs inside the guest and carries out actions.
 
+df/
+        'virt-df' command and documentation.
+
 examples/
         The examples.