io: fix possible double free of task error object

If a QIOTask has an error set and the calling code uses
qio_task_propagate_error() to steal the reference to
that Error object, the task would not clear its own
reference. This would lead to a double-free when
qio_task_free runs, if the caller had (correctly) freed
the Error object they now owned.

Signed-off-by: Daniel P. Berrange <berrange@redhat.com>