docs/qapi-domain: warn when QAPI domain xrefs fail to resolve

docs/qapi-domain: warn when QAPI domain xrefs fail to resolve

This patch adds a warning (which is a build failure under our current
build settings) whenever a QAPI cross-reference fails to resolve.

This applies to any cross-references of the form :qapi:{role}:`foo`,
which covers all of the automatically generated references by the qapi
domain, and any such references that are manually written into the
documentation rst files.

Cross-references of the form `foo` do not use this system, but are
already configured to issue a warning (Again, a build failure) if the
cross-reference isn't found anywhere.

Adds warnings that look like the following:

docs/qapi/index.rst:48: WARNING: qapi:type reference target not found: 'footype' [ref.qapi]
docs/qapi/index.rst:50: WARNING: qapi:mod reference target not found: 'foomod' [ref.qapi]

Signed-off-by: John Snow <jsnow@redhat.com>
Message-ID: <20250311034303.75779-31-jsnow@redhat.com>
Acked-by: Markus Armbruster <armbru@redhat.com>
Signed-off-by: Markus Armbruster <armbru@redhat.com>