qom: Don't make link NULL on object_property_set_link() failure

qom: Don't make link NULL on object_property_set_link() failure

The error behavior of object_property_set_link() is dangerous.  It sets
the link property object to NULL if an error occurs.  A setter function
should either succeed or fail, it shouldn't leave the value NULL on
failure.

Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
Reviewed-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Andreas Färber <afaerber@suse.de>