misc: encode the Emacs suggstions from HACKING into a .dir-locals.el file

With this file in place, opening any source file in libvirt will set up
Emacs for proper indentation.

.dir-locals.el

+(
+ (c-mode . (
+            (c-file-style . "K&R")
+            (indent-tabs-mode . nil)
+            (c-indent-level . 4)
+            (c-basic-offset . 4)
+            ))
+ )