ptp: Add vDSO-style vmclock support

ptp: Add vDSO-style vmclock support

The vmclock "device" provides a shared memory region with precision clock
information. By using shared memory, it is safe across Live Migration.

Like the KVM PTP clock, this can convert TSC-based cross timestamps into
KVM clock values. Unlike the KVM PTP clock, it does so only when such is
actually helpful.

The memory region of the device is also exposed to userspace so it can be
read or memory mapped by application which need reliable notification of
clock disruptions.

Signed-off-by: David Woodhouse <dwmw@amazon.co.uk>