qemu-common.h: Comment about usage rules

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: Eduardo Habkost <ehabkost@redhat.com>
Signed-off-by: Andreas Färber <afaerber@suse.de>