The ``loaded=on`` option in the command line or QMP ``object-add`` either had
no effect (if ``loaded`` was the last option) or caused options to be
effectively ignored as if they were not given. The property is therefore
useless and was deprecated in 6.0; make it read-only now.
The patch is best reviewed with "-b".
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>