qapi: Use quotes more consistently in frontend error messages

Consistently enclose error messages in double quotes.  Use single
quotes within, except for one case of "'".

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Message-Id: <20190914153506.2151-8-armbru@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
20 files changed