CODINGSTYLE.md updates

Add to our coding conventions, CODINGSTYLE.md, three coding conventions
which were mentioned by Avi Kivity in a review, and also fit most of our
code. These conventions are:

 1. No (void) in function declarations in C++ code.
 2. No space between function name and argument list.
 3. No parantheses around return value.

Signed-off-by: Nadav Har'El <nyh@cloudius-systems.com>
Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>