clock: relative-time clock_event::set()

OSv's timer mechanism hinges on the Local APIC's (per-cpu) one-shot timer,
which delivers an interrupt after the requested number of nanoseconds.

The API to set this timer, clock_event::set(), took the absolute time of
the next interrupt. However, what it really needs is the duration in
nanoseconds until the next interrupt.

So this patch we change the basic clock_event::set() API to take a
duration, and implement the original clock_event::set(s64) - taking an s64
absolute wall-clock time - as a simple wrapper.  The next patch will add
more wrappers for set() taking absolute times from different clocks.
Later patches in this series will stop using the old set(s64) version,
until it is dropped in the end of the series.

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>