autotest: Let atest host migrate respect board & model correctly.

BUG=chromium:914467
TEST=Ran "atest host migrate --board=coral --model=astronaut
--pool=bvt --env=prod --migration"

Change-Id: Ia68d71f388ea041c385658bcca7d69184ea95b6e
Reviewed-on: https://chromium-review.googlesource.com/1376892
Commit-Ready: ChromeOS CL Exonerator Bot <chromiumos-cl-exonerator@appspot.gserviceaccount.com>
Tested-by: Xixuan Wu <xixuan@chromium.org>
Reviewed-by: Allen Li <ayatane@chromium.org>
diff --git a/cli/host.py b/cli/host.py
index a80bc75..25584de 100644
--- a/cli/host.py
+++ b/cli/host.py
@@ -1296,8 +1296,10 @@
 
         label_map = None
         labels = []
+        if self.board:
+            labels.append('board:%s' % self.board)
         if self.model:
-            labels.append('board:%s' % self.model)
+            labels.append('model:%s' % self.model)
         if self.pool:
             labels.append('critical_pool:%s' % self.pool)
         if labels: