New tool: virt-edit

Edit any file in a guest.  This was possibly previously
using guestfish, but having a separate command makes it
simpler.

The usage is simply:

  virt-edit mydomain /some/file

It runs $EDITOR or vi on the file, and if the user changes
it, uploads the result back to the VM.

HACKING

@@ -83,6 +83,9 @@ daemon/
 df/
         The virt-df tool.
 
+edit/
+	The virt-edit tool.
+
 examples/
         The examples.