kvm tools: Add PPC64 XICS interrupt controller support

This patch adds XICS emulation code (heavily borrowed from QEMU), and wires
this into kvm_cpu__irq() to fire a CPU IRQ via KVM.  A device tree entry is
also added.  IPIs work, xics_alloc_irqnum() is added to allocate an external
IRQ (which will later be used by the PHB PCI code) and finally, kvm__irq_line()
can be called to raise an IRQ on XICS.

Signed-off-by: Matt Evans <matt@ozlabs.org>
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
Signed-off-by: Pekka Enberg <penberg@kernel.org>