Migrate vector timers to new timer API

The patches for the UML vector drivers were in-flight when
the timer changes happened and were not covered by them.

This change migrates vector_kern.c to use the new timer API.

Signed-off-by: Anton Ivanov <anton.ivanov@cambridgegreys.com>
Signed-off-by: Richard Weinberger <richard@nod.at>