log: tests: fix appending the virt-log output

virt-log uses /dev/stdout as output file for guestfs_download, which
will open the output file in truncate mode, truncating the output of
test-virt-log.sh.

Make sure to redirect the output of virt-log so it does not clobber the
test output.

Thanks to Richard for the hint.

cat/test-virt-log.sh

@@ -45,6 +45,6 @@ for f in ../tests/guests/{fedora,debian,ubuntu}.img; do
         echo
         continue
     fi
-    $VG virt-log -a "$f"
+    $VG virt-log -a "$f" |& cat
     echo
 done