[kernel][hypervisor] KVM clock support in hypervisor

KVM/Xen paravirtualized clock API consists of monotonic clock
source (we refer to it as system time) and fixed point in time
(we refer to it as boot time), and boot time + system time should
give current wall time. Since system time is supposed to be
monotonic it means that boot time may be adjusted (for example,
by NTP) and therefore we use sys_get_clock to get boot time.

System time is exported per VCPU and periodically updated by host.
TSC is used to adjust system time value in the guest between
those updates. We update system time on VCPU resume.

Getting system time in the guest is expected to be fast, but boot
time on the other hand is not expected to be fast and only
guaranteed to be updated by host after MSR write from guest and
therefore incurs vmexit. Despite the name boot time doesn't need
to be guest boot time. In KVM it's actually host boot time and we
use the same boot time for this implementation.

And one comment regarding naming. KVM and Xen use the same
structures layout for paravirtualized clock, but KVM uses MSRs
to communicate with guests and Xen uses PV ops for that. So data
strcutures are common and use prefix pv or Pv, while constants
describing MSRs and KVM specific flags use Kvm prefix.

Change-Id: I81cdbd893985ad17bcf5f163e652ec1bb63feacb

[x86] Move hypervisor assembly to hypervisor dir.

Also rename the file to vmx.S. This is to mirror the upcoming changes
to the arm64 side.

Change-Id: Id97ecbc71b0f7d5fc7fd2371a8e79b32ade6003b

[hypervisor] Invert build dependencies.

This allows us to depend on lib/magenta from lib/hypervisor, so we can remove
all of the WITH_LIB_MAGENTA defines and still build cleanly with the -test
targets.

Change-Id: I7a54c2ae4d812213a21ff67b963626a65879b2ea