clock: add clock_event::set(timepoint)

In the previous patch we built clock_event::set(s64) on top of
set(duration). In this patch we add set(timepoint), for absolute time
in any clock supporting the now() static method. In the following patches,
we'll convert our timers to using the monotonic clock osv::clock::uptime,
and use timepoints of that clock in calls to clock_event::set().

By temporarily having both s64 and std::chrono:time_point versions
of set(), we make this patch series bisectable. At the end of this series,
the s64 variant will be removed.

Reviewed-by: Glauber Costa <glommer@cloudius-systems.com>
Signed-off-by: Nadav Har'El <nyh@cloudius-systems.com>
Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>

drivers/clockevent.hh

@@ -29,6 +29,14 @@ public:
         s64 now = clock::get()->time();
         set(std::chrono::nanoseconds(time - now));
     }
+    // Can be used on a std::chrono::time_point of a clock which supports
+    // the now() method. For example osv::clock::uptime::time_point.
+    template<class timepoint>
+    inline void set(timepoint time) {
+        auto now = timepoint::clock::now();
+        using namespace std::chrono;
+        set(duration_cast<nanoseconds>(time - now));
+    }
     void set_callback(clock_event_callback* callback);
     clock_event_callback* callback() const;
 protected: