qemu/qmp/legacy: Replace 'returns-whitelist' with the correct type
'returns-whitelist' has been renamed to 'command-returns-exceptions' in QEMU commit b86df3747848 ("qapi: Rename pragma *-whitelist to *-exceptions").
'returns-whitelist' has been renamed to 'command-returns-exceptions' in QEMU commit b86df3747848 ("qapi: Rename pragma *-whitelist to *-exceptions").