Add .dir-locals.el file to configure emacs coding style

Some default emacs setups indent by 2 spaces and uses tabs
which is counter to the QEMU coding style rules. Adding a
.dir-locals.el file in the top level of the GIT repo will
inform emacs about the QEMU coding style, and so assist
contributors in avoiding common style mistakes before
they submit patches.

Signed-off-by: Daniel P. Berrange <berrange@redhat.com>
Reviewed-by: Markus Armbruster <armbru@redhat.com>
Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>