xen/events: bind all new interdomain events to VCPU0

Commit fc087e10 (xen/events: remove
unnecessary init_evtchn_cpu_bindings()) causes a regression.

The kernel-side VCPU binding was not being correctly set for newly
allocated or bound interdomain events.  In ARM guests where 2-level
events were used, this would result in no interdomain events being
handled because the kernel-side VCPU masks would all be clear.

x86 guests would work because the irq affinity was set during irq
setup and this would set the correct kernel-side VCPU binding.

Fix this by properly initializing the kernel-side VCPU binding in
bind_evtchn_to_irq().

Reported-and-tested-by: Julien Grall <julien.grall@linaro.org>
Signed-off-by: David Vrabel <david.vrabel@citrix.com>
Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>