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/hw/9pfs/9p-proxy.c b/hw/9pfs/9p-proxy.c
index f030c6a..0f288b1 100644
--- a/hw/9pfs/9p-proxy.c
+++ b/hw/9pfs/9p-proxy.c
@@ -9,10 +9,12 @@
  * This work is licensed under the terms of the GNU GPL, version 2.  See
  * the COPYING file in the top-level directory.
  */
+
 #include "qemu/osdep.h"
 #include <sys/socket.h>
 #include <sys/un.h>
 #include "9p.h"
+#include "qapi/error.h"
 #include "qemu/cutils.h"
 #include "qemu/error-report.h"
 #include "fsdev/qemu-fsdev.h"