xen/intr: balance dynamic interrupts across available vCPUs

By default Xen binds all event channels to vCPU#0, and FreeBSD only shuffles
the interrupt sources once, at the end of the boot process. Since new event
channels might be created after this point (because new devices or backends
are added), try to automatically shuffle them at creation time.

This does not affect VIRQ or IPI event channels, that are already bound to a
specific vCPU as requested by the caller.

Sponsored by: Citrix Systems R&D