For configurations with multiple xenhosts, we need to handle events
generated from multiple xenhosts.
Having more than one upcall handler might be quite hairy, and it would
be simpler if the callback from L0-Xen could be bounced via L1-Xen.
This will also mean simpler pv_irq_ops code because now the IF flag
maps onto the xh_default->vcpu_info->evtchn_upcall_mask.
However, we still update the xh_remote->vcpu_info->evtchn_upcall_mask
on a best effort basis to minimize unnecessary work in remote xenhost.