qemu-common.h: Comment about usage rules

Every time we make a tiny change on a header file, we often find
circular header dependency problems. To avoid this nightmare, we need to
stop including qemu-common.h from other headers, and we should gradually
move the declarations from the catch-all qemu-common.h header to their
specific headers.

This simply adds a comment documenting the rules about qemu-common.h,
hoping that people will see it before including qemu-common.h from other
header files, and before adding more declarations to qemu-common.h.
Signed-off-by: NEduardo Habkost <ehabkost@redhat.com>
Signed-off-by: NAndreas Färber <afaerber@suse.de>

qemu-common.h

 
-/* Common header file that is included by all of qemu.  */
+/* Common header file that is included by all of QEMU.
+ *
+ * This file is supposed to be included only by .c files. No header file should
+ * depend on qemu-common.h, as this would easily lead to circular header
+ * dependencies.
+ *
+ * If a header file uses a definition from qemu-common.h, that definition
+ * must be moved to a separate header file, and the header that uses it
+ * must include that header.
+ */
 #ifndef QEMU_COMMON_H
 #define QEMU_COMMON_H