powerpc/irq: Add support for HV virtualization interrupts

This will be delivering external interrupts from the XIVE to the
Hypervisor. We treat it as a normal external interrupt for the
lazy irq disable code (so it will be replayed as a 0x500) and
route it to do_IRQ.

Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>