conf: make virstoragetest debug easier

I'm tired of alternating between test failures due to bugs in
my refactoring work, vs. test failures due to leftovers in
the file system from the previous test.  This patch has no
impact when the testsuite is successful, but doeesn't hurt either.

* tests/virstoragetest.c (testPrepImages): Clean up from prior
failed test.
Signed-off-by: NEric Blake <eblake@redhat.com>

tests/virstoragetest.c

@@ -102,6 +102,9 @@ testPrepImages(void)
     if (!qemuimg)
         goto skip;
 
+    /* Clean up from any earlier failed tests */
+    virFileDeleteTree(datadir);
+
     /* See if qemu-img supports '-o compat=xxx'.  If so, we force the
      * use of both v2 and v3 files; if not, it is v2 only but the test
      * still works. */