Clean up test root before running test

The leftover files under /data/local/tmp/virt (especially instance.img)
sometimes cause test failure.

Test: atest ComposHostTestCases MicrodroidHostTestCases
Change-Id: Id8804a9e24d2a0cee17cc28e7b5bb8a0d300b8e7
diff --git a/tests/hostside/helper/java/android/virt/test/VirtualizationTestCaseBase.java b/tests/hostside/helper/java/android/virt/test/VirtualizationTestCaseBase.java
index 97cd426..4c8f5eb 100644
--- a/tests/hostside/helper/java/android/virt/test/VirtualizationTestCaseBase.java
+++ b/tests/hostside/helper/java/android/virt/test/VirtualizationTestCaseBase.java
@@ -61,6 +61,9 @@
 
         // disconnect from microdroid
         tryRunOnHost("adb", "disconnect", MICRODROID_SERIAL);
+
+        // remove any leftover files under test root
+        android.tryRun("rm", "-rf", TEST_ROOT + "*");
     }
 
     public static void cleanUpVirtualizationTestSetup(ITestDevice androidDevice)