Initialize kvmclock base, on kvmclock system MSR write time,
so that the guest sees kvmclock counting from zero.

This matches baremetal behaviour when kvmclock in guest
sets sched clock stable.
Signed-off-by: Marcelo Tosatti <mtosatti@redhat.com>
[Remove unnecessary comment. - Paolo]
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>