Make ipi_cpu() function as intended.

IPI's in Xen are implemented through hypervisor event channels.
The MP code creates a pair of IRQs for each base IPI per CPU
(one for IPI function dispatch calls, one for IPI bitmap dispatch calls.)
Using PCPU_GET() was returning the IRQ of the IPI handler for the
current CPU; thus calls to ipi_cpu() were sending itself a message.
Instead, looking up the IPI in the target CPU ipi-to-irq map is needed.

Note: This doesn't fix Xen SMP (far from it!) but it at least
sends IPI's to the right places. Next - sending IPIs..

PR: 135069