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/numa.c b/numa.c
index a9528aa..7e0e789 100644
--- a/numa.c
+++ b/numa.c
@@ -30,6 +30,7 @@
 #include "qom/cpu.h"
 #include "qemu/error-report.h"
 #include "qapi-visit.h"
+#include "qapi/error.h"
 #include "qapi/opts-visitor.h"
 #include "hw/boards.h"
 #include "sysemu/hostmem.h"