xen_hvc: no reason to write the type key on xenstore

The backend type is chosen by the toolstack. Regardless, the frontend
should not care, especially given that it cannot have an effect as the
backend is started before this code is run.

Signed-off-by: Stefano Stabellini <stefano.stabellini@eu.citrix.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>