timer: Move NANOSECONDS_PER_SECONDS to timer.h

We want to be able to reuse this define by making it common to
multiple QEMU modules.

This also makes it an integer since there's no need for it to be a
float.

Signed-off-by: Alberto Garcia <berto@igalia.com>
Message-id: 6375912849da2ab561046dd013684535ccecca44.1434113783.git.berto@igalia.com
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>