xen: do not build backends for targets that do not support xen

xen: do not build backends for targets that do not support xen

Change Makefile.objs to use CONFIG_XEN instead of CONFIG_XEN_BACKEND, so
that the Xen backends are only built for targets that support Xen.

Set CONFIG_XEN in the toplevel Makefile to ensure that files that are
built only once pick up Xen support properly.

Signed-off-by: Stefano Stabellini <stefano@aporeto.com>
Tested-by: Greg Kurz <groug@kaod.org>
Reviewed-by: Greg Kurz <groug@kaod.org>
CC: pbonzini@redhat.com
CC: peter.maydell@linaro.org
CC: rth@twiddle.net
CC: stefanha@redhat.com
Message-Id: <1489694518-16978-1-git-send-email-sstabellini@kernel.org>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>