[kernel][pmm] change pmm single page alloc routines to return status

Move from returning a status and the page pointer and/or physical
address in an argument.

Tested: build and runtests on both x86 and arm

Change-Id: If10d877f152d355bfa1359338cd71f5aefbccfb4

[x86][pvclock] Fix hang on KVM

Zero the pages we give to the hypervisor for the clock datastructures.
Some versions of KVM do not assign an initial value to the version field,
so if it starts with an odd value, we will just hang.

ZX-2110 #done

Change-Id: I8a2678e9741c1e2abc45f97c96fdb927037bcb25

[pvclock] Set and test for stable clocksource

This allows us to query for whether the pvclock is stable, and then use
that to decide whether to default to TSC as the wall-clock.

Change-Id: I564df4537466b5eac4391a67671e88ba1e7a2ba4

[x86] Print a message when getting the TSC freq from pvclock

Change-Id: I29a46804389252bb34305f69ed796f72d0aa9208

[kernel][timer] Calibrate TSC with paravirtualized clock

Adds some support for KVM/PV clock. Currently only calibrating the TSC
via the information in the system time struct. This allows the
hypervisor to host Zircon while nested in KVM without implementing the
PIT timer.

Change-Id: I63f0961112b56dd09b1bc893b74f2646d844d7aa