This just moves the timer-arming logic from xen_initclocks into
xen_resumeclocks so that it runs on resume too.
I hypothesize that this is necessary for Xen to resume. Otherwise,
how could the one-shot timer possibly by rearmed? On the other hand,
it is conceivable that something automatically rearms it.
This also reorders the initialization so that we establish a timer
interrupt handler first, and _then_ arm the timer. If the order
matters, it is hard to imagine that the other way is correct:
conceivably, the interrupt could arrive before we've established the
handler, and then there's nothing to rearm it.
Whether this is _sufficient_ for Xen to resume, I don't know.
Symptoms recently reported in
<https://mail-index.netbsd.org/port-xen/2018/06/15/msg009207.html>
look different from how I would expect this to manifest, which is as
a system wedged because there's no no hardclock activity.
ok cherry
1cf40ee15d