kvmclock: serialize RDTSC

Serialize RDTSC so its executed inside kvmclock_read
section.

Fixes https://bugzilla.redhat.com/show_bug.cgi?id=922285



Signed-off-by: Marcelo Tosatti <mtosatti@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>