CODING_STYLE: Section about conditional statement

Yoda conditions lack readability, and QEMU has a
strict compiler configuration for checking a common
mistake like "if (dev = NULL)". Make it a written rule.

Signed-off-by: Gonglei <arei.gonglei@huawei.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>
