crypto: make loaded property read-only

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>