(virq_timer_to_evtch, indexed by cpuid) different from the
VIRQ <> event channel one (virq_to_evtch, indexed by event channel ID).
This is fine: fix a "harmless" bug that resulted in the event
channel of VIRQ_TIMER getting lost during bind as it was not stored
in the proper array.
"Harmless" because it is not critical for -current, however in the Xen
save/restore branch this completely cripples restore. Xen clock gets
suspended, but never comes back (fetched channel ID being invalid). Oops.
Add a small comment so we can better see the "get => allocate? => set"
chain of actions when binding/unbinding event channels.