add editorconfig

Add a .editorconfig file for qemu.  Specifies the indent and tab style
for various files (C code and Makefiles for starters).  Most popular
editors support this either natively or via plugin.

Check http://editorconfig.org/ for details.

Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
Message-id: 20170717101547.22295-1-kraxel@redhat.com
