qapi: add cross-references to migration.json

qapi: add cross-references to migration.json

Note that a reference to MIGRATION needs to be disambiguated with a
:qapi:event: prefix.  Without this, Sphinx complains

    more than one target found for 'any' cross-reference 'MIGRATION': could be :std:ref:`Migration framework` or :qapi:event:`QMP:migration.MIGRATION`

Signed-off-by: John Snow <jsnow@redhat.com>
Message-ID: <20250711054005.60969-9-jsnow@redhat.com>
Reviewed-by: Markus Armbruster <armbru@redhat.com>
[Commit message amended to explain need for :qapi:event:]
Signed-off-by: Markus Armbruster <armbru@redhat.com>