pvclock: reuse pvclock's functionality to convert tsc to nano

This patch provides a way to, given a measurement from tsc, acquire a nanosecond
figure. It works only for xen and kvm pvclocks, and I intend to use it for acquiring
early boot figures.

It is possible to measure the tsc frequency and with that figure out how to
convert a tsc read to nanoseconds, but I don't think we should pay that price.
Most of the pvclock drivers already provide that functionality, and we are not
planning that many users of that interface anyway.

Signed-off-by: Glauber Costa <glommer@cloudius-systems.com>
Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>