hw/xen: Implement EVTCHNOP_bind_virq

hw/xen: Implement EVTCHNOP_bind_virq

Add the array of virq ports to each vCPU so that we can deliver timers,
debug ports, etc. Global virqs are allocated against vCPU 0 initially,
but can be migrated to other vCPUs (when we implement that).

The kernel needs to know about VIRQ_TIMER in order to accelerate timers,
so tell it via KVM_XEN_VCPU_ATTR_TYPE_TIMER. Also save/restore the value
of the singleshot timer across migration, as the kernel will handle the
hypercalls automatically now.

Signed-off-by: David Woodhouse <dwmw@amazon.co.uk>