VMClock support

[dwmw2]

The VMClock device is primarily designed for a hypervisor to advertise precision clock information to its guests — so that they don't all have to repeat the same work of calibrating the same underlying oscillator which drives the host CPU's TSC or other hardware counter.

Essentially, it just provides a shared memory region which tells the (guest) operating system the relationship between the hardware counter and real time.

With PCIe PTM, we could provide that same data structure in the BAR of a PCI device....