clock: Use new clock APIs in callout implementation

Change callout implementation to use the new <osv/clock.hh> APIs and the
monotonic clock.

Since _callout.h now uses the C++ type osv::clock::uptime::time_point,
it can only be used from C++ code. All the relevant code is already C++.

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>