Include qapi/error.h exactly where needed

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 <[email protected]>
Reviewed-by: Philippe Mathieu-Daudé <[email protected]>
Signed-off-by: Markus Armbruster <[email protected]>
Message-Id: <20180201111846[email protected]>
[Semantic conflict with commit 34e304e975 resolved, OSX breakage fixed]