Include qapi/error.h exactly where needed

This cleanup makes the number of objects depending on qapi/error.h
drop from 1910 (out of 4743) to 1612 in my "build everything" tree.

While there, separate #include from file comment with a blank line,
and drop a useless comment on why qemu/osdep.h is included first.

Reviewed-by: Eric Blake <eblake@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Signed-off-by: Markus Armbruster <armbru@redhat.com>
Message-Id: <20180201111846.21846-5-armbru@redhat.com>
[Semantic conflict with commit 34e304e975 resolved, OSX breakage fixed]
diff --git a/chardev/char-ringbuf.c b/chardev/char-ringbuf.c
index df52b04..c1fa4c3 100644
--- a/chardev/char-ringbuf.c
+++ b/chardev/char-ringbuf.c
@@ -21,9 +21,11 @@
  * OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
  * THE SOFTWARE.
  */
+
 #include "qemu/osdep.h"
 #include "chardev/char.h"
 #include "qmp-commands.h"
+#include "qapi/error.h"
 #include "qemu/base64.h"
 
 /* Ring buffer chardev */