spapr/vio: deprecate the "irq" property

VIO devices have an "irq" property that can be used by the sPAPR IRQ
allocator as an IRQ number hint. But it is not set in QEMU nor in
libvirt. It brings unnecessary complexity to the underlying layers
managing the IRQ number space and it is in full opposition with the
new static IRQ allocator we want to introduce in sPAPR.

Let's deprecate it to simplify the spapr_irq_alloc routine in the
future.

Signed-off-by: Cédric Le Goater <clg@kaod.org>
Reviewed-by: Thomas Huth <thuth@redhat.com>
Reviewed-by: Greg Kurz <groug@kaod.org>
[dwg: Check qtest_enabled() to suppress bogus warnings from make check]
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>