add qemu_get_clock_ns

Some places use get_clock directly because they want to access the
rt_clock with nanosecond precision.  Add a function to do exactly that
instead of using internal interfaces.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>
diff --git a/qemu-timer.h b/qemu-timer.h
index e7eaa04..c17b4e6 100644
--- a/qemu-timer.h
+++ b/qemu-timer.h
@@ -25,6 +25,7 @@
 extern QEMUClock *host_clock;
 
 int64_t qemu_get_clock(QEMUClock *clock);
+int64_t qemu_get_clock_ns(QEMUClock *clock);
 
 QEMUTimer *qemu_new_timer(QEMUClock *clock, QEMUTimerCB *cb, void *opaque);
 void qemu_free_timer(QEMUTimer *ts);