v2v: Add -o discard option when fstrimming.

This *may* be required by some filesystems in order for fstrim to
work.  I'm not actually sure if this is true, but it's what
virt-sparsify --in-place does, and that utility has been tested a lot
more in regards to trimming.

(cherry picked from commit dd22e37)

v2v/v2v.ml

@@ -421,7 +421,10 @@ and do_fstrim g no_trim inspect =
   List.iter (
     fun dev ->
       g#umount_all ();
-      let mounted = try g#mount dev "/"; true with G.Error _ -> false in
+      let mounted =
+        try g#mount_options "discard" dev "/"; true
+        with G.Error _ -> false in
+
       if mounted then (
         try g#fstrim "/"
         with G.Error msg ->