rtc: add -rtc clock=rt

This will let people use backwards-compatible semantics for devices that
will be affected by the following patches.

Signed-off-by: Paolo Bonzini <[email protected]>
Signed-off-by: Peter Maydell <[email protected]>
diff --git a/vl.c b/vl.c
index 0fccf50..71a4755 100644
--- a/vl.c
+++ b/vl.c
@@ -529,6 +529,8 @@
     if (value) {
         if (!strcmp(value, "host")) {
             rtc_clock = host_clock;
+        } else if (!strcmp(value, "rt")) {
+            rtc_clock = rt_clock;
         } else if (!strcmp(value, "vm")) {
             rtc_clock = vm_clock;
         } else {