tests: virstorage: Fix backing file format of created image

We create some images for testing our code. We've recorded wrong format
of the backing file for one of the images though.
Signed-off-by: NPeter Krempa <pkrempa@redhat.com>
Reviewed-by: NJán Tomko <jtomko@redhat.com>

tests/virstoragetest.c

@@ -964,7 +964,7 @@ mymain(void)
     /* Rewrite wrap and qcow2 back to 3-deep chain, absolute backing */
     virCommandFree(cmd);
     cmd = virCommandNewArgList(qemuimg, "rebase", "-u", "-f", "qcow2",
-                               "-F", "qcow2", "-b", absraw, "qcow2", NULL);
+                               "-F", "raw", "-b", absraw, "qcow2", NULL);
     if (virCommandRun(cmd, NULL) < 0)
         ret = -1;