Add a clocksource ID for the x86 kvmclock.

Also, for ptp_kvm, set the recently added struct system_counterval_t member
cs_id to the clocksource ID (x86 kvmclock or ARM Generic Timer). In the
future, get_device_system_crosststamp() will compare the clocksource ID in
struct system_counterval_t, rather than the clocksource.

For now, to avoid touching too many subsystems at once, extract the
clocksource ID from the clocksource. The clocksource dereference will be
removed once everything is converted over..
Signed-off-by: Peter Hilber <peter.hilber@opensynergy.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Link: https://lore.kernel.org/r/20240201010453.2212371-5-peter.hilber@opensynergy.com