qcrypto_secret_prop_set_loaded() forgets to reset secret->rawdata after
unloading a secret, which will lead to a double free at some point.

Because there is no use case for unloading an already loaded secret
(apart from deleting the whole secret object) and we know that nobody
could use this because it would lead to crashes, let's just forbid the
operation instead of fixing the unloading.

Eventually, we'll want to get rid of 'loaded' in the external interface,
but for the meantime this is more consistent with rng, which has a
similar property 'opened' that also can't be reset to false after it
became true.

Signed-off-by: Kevin Wolf <kwolf@redhat.com>
Signed-off-by: Daniel P. Berrangé <berrange@redhat.com>