docs/qapi-domain: add XREF compatibility goop for Sphinx < 4.1

docs/qapi-domain: add XREF compatibility goop for Sphinx < 4.1

Sphinx < 4.1 handles cross-references ... differently. Factor out and
isolate the compatibility goop we need to make cross references work
properly in old versions of Sphinx.

Yes, it's ugly. Yes, it works. No, I don't want to talk about
it.

Understand that this patch exists because of the overflowing love in my
heart.

Signed-off-by: John Snow <jsnow@redhat.com>
Message-ID: <20250311034303.75779-30-jsnow@redhat.com>
Acked-by: Markus Armbruster <armbru@redhat.com>
Signed-off-by: Markus Armbruster <armbru@redhat.com>