CONFIG_XEN_PV breaks xen_create_contiguous_region on ARM

xen_create_contiguous_region has now only an implementation if
CONFIG_XEN_PV is defined. However, on ARM we never set CONFIG_XEN_PV but
we do have an implementation of xen_create_contiguous_region which is
required for swiotlb-xen to work correctly (although it just sets
*dma_handle).

Cc: <stable@vger.kernel.org> # 4.12
Fixes: 16624390 ("xen: create xen_create/destroy_contiguous_region() stubs for PVHVM only builds")
Signed-off-by: Stefano Stabellini <stefanos@xilinx.com>
Reviewed-by: Juergen Gross <jgross@suse.com>
CC: Jeff.Kubascik@dornerworks.com
CC: Jarvis.Roach@dornerworks.com
CC: Nathan.Studer@dornerworks.com
CC: vkuznets@redhat.com
CC: boris.ostrovsky@oracle.com
CC: jgross@suse.com
CC: julien.grall@arm.com
Signed-off-by: Juergen Gross <jgross@suse.com>