Include qapi/qmp/qdict.h exactly where needed

This cleanup makes the number of objects depending on qapi/qmp/qdict.h
drop from 4550 (out of 4743) to 368 in my "build everything" tree.
For qapi/qmp/qobject.h, the number drops from 4552 to 390.

While there, separate #include from file comment with a blank line.

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: <20180201111846.21846-13-armbru@redhat.com>
diff --git a/blockdev.c b/blockdev.c
index 90ef316..bdbdeae 100644
--- a/blockdev.c
+++ b/blockdev.c
@@ -40,6 +40,7 @@
 #include "qemu/error-report.h"
 #include "qemu/option.h"
 #include "qemu/config-file.h"
+#include "qapi/qmp/qdict.h"
 #include "qapi/qmp/qnum.h"
 #include "qapi/qmp/qstring.h"
 #include "qapi-visit.h"