xen/arm: send IPIs to inject irqs into guest vcpus running on different pcpus

xen/arm: send IPIs to inject irqs into guest vcpus running on different pcpus

If we need to inject an irq into a VCPU that is running on a different
processor, we shouldn't just enqueue the irq into the lr_pending and
inflight lists and wait for something to interrupt the guest execution.
Send an IPI to the target pcpu so that Xen can inject the new interrupt
returning to guest.

Signed-off-by: Stefano Stabellini <stefano.stabellini@eu.citrix.com>
Acked-by: Ian Campbell <ian.campbell@citrix.com>