io: stop incrementing reference in qio_task_get_source

Incrementing the reference in qio_task_get_source is
not necessary, since we're not running concurrently
with any other code touching the QIOTask. This
minimizes chances of further memory leaks.

Reviewed-by: Eric Blake <eblake@redhat.com>
Signed-off-by: Daniel P. Berrange <berrange@redhat.com>
