qapi: Require ASCII in schema

Python 2 and Python 3 have a wild history of whether strings
default to ascii or unicode, where Python 3 requires checking
isinstance(foo, basestr) to cover all strings, but where that
code is not portable to Python 2.  It's simpler to just state
that we don't care about Unicode strings, and to just always
use the simpler isinstance(foo, str) everywhere.

I'm no python expert, so I'm basing it on this conversation:
https://lists.gnu.org/archive/html/qemu-devel/2014-09/msg05278.html

Signed-off-by: Eric Blake <eblake@redhat.com>
Reviewed-by: Markus Armbruster <armbru@redhat.com>
Signed-off-by: Markus Armbruster <armbru@redhat.com>
