qapi: support updating expected test output via make

qapi: support updating expected test output via make

It is possible to pass --update to tests/qapi-schema/test-qapi.py
to make it update the output files on error. This is inconvenient
to achieve though when test-qapi.py is run indirectly by make/meson.

Instead simply allow for an env variable to be set:

 $ QAPI_TEST_UPDATE= make check-qapi-schema

Signed-off-by: Daniel P. Berrangé <berrange@redhat.com>
Message-Id: <20230420102619.348173-2-berrange@redhat.com>
Reviewed-by: Markus Armbruster <armbru@redhat.com>
Signed-off-by: Markus Armbruster <armbru@redhat.com>