docs/sphinx: change default role to "any"

This interprets single-backtick syntax in all of our Sphinx docs as a
cross-reference to *something*, including Python symbols.

From here on out, new uses of `backticks` will cause a build failure if
the target cannot be referenced.

Signed-off-by: John Snow <jsnow@redhat.com>
Reviewed-by: Eduardo Habkost <ehabkost@redhat.com>
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Message-Id: <20211004215238.1523082-4-jsnow@redhat.com>

docs/conf.py

@@ -85,6 +85,11 @@
 # The master toctree document.
 master_doc = 'index'
 
+# Interpret `single-backticks` to be a cross-reference to any kind of
+# referenceable object. Unresolvable or ambiguous references will emit a
+# warning at build time.
+default_role = 'any'
+
 # General information about the project.
 project = u'QEMU'
 copyright = u'2021, The QEMU Project Developers'