include/hw/xen/xen_common: return error from xen_create_ioreq_server

include/hw/xen/xen_common: return error from xen_create_ioreq_server

This is done to prepare for enabling xenpv support for ARM architecture.
On ARM it is possible to have a functioning xenpv machine with only the
PV backends and no IOREQ server. If the IOREQ server creation fails,
continue to the PV backends initialization.

Signed-off-by: Stefano Stabellini <stefano.stabellini@amd.com>
Signed-off-by: Vikram Garhwal <vikram.garhwal@amd.com>
Reviewed-by: Stefano Stabellini <sstabellini@kernel.org>
Reviewed-by: Paul Durrant <paul@xen.org>