kvmclock: Implement support for old kvmclock MSRs.

Older versions of KVM and user VMMs expose kvmclock MSRs at different
MSR offsets. Detect the old flag in kvmclock::probe() and use the old
MSRs if they are the only ones available.

Signed-off-by: Venkatesh Srinivas <venkateshs@google.com>
Signed-off-by: Glauber Costa <glommer@cloudius-systems.com>