qapi2texi: Plainer enum value and member name formatting

qapi2texi: Plainer enum value and member name formatting

Use @code{%s} instead of @code{'%s'}.  Impact, using @id as example:

* Texinfo
  -@item @code{'id'}
  +@item @code{id}

* HTML
  -<dt><code>'id'</code></dt>
  +<dt><code>id</code></dt>

* POD (for manual pages):
  -=item C<'id'>
  +=item C<id>

* Formatted manual pages:
  -'id'
  +"id"

* Plain text:
  -     ''id''
  +     'id'

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Message-Id: <1489582656-31133-21-git-send-email-armbru@redhat.com>