xics: fix several error leaks

If object_property_get_link() fails then it allocates an error, which
must be freed before returning. The error_get_pretty() function is
merely an accessor to the error message and doesn't free anything.

The error.h header indicates how to do it right:

 * Pass an existing error to the caller with the message modified:
 *     error_propagate(errp, err);
 *     error_prepend(errp, "Could not frobnicate '%s': ", name);

Signed-off-by: Greg Kurz <groug@kaod.org>
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
