x86: IRQ affinity should track vCPU affinity

x86: IRQ affinity should track vCPU affinity

With IRQs getting bound to the CPU the binding vCPU currently runs on
there can result quite a bit of extra cross CPU traffic as soon as
that vCPU moves to a different pCPU. Likewise, when a domain re-binds
an event channel associated with a pIRQ, that IRQ's affinity should
also be adjusted.

The open issue is how to break ties for interrupts shared by multiple
domains - currently, the last request (at any point in time) is being
honored.

Signed-off-by: Jan Beulich <jbeulich@novell.com>