udev: fix block devices ignoring rule

First of all there was missing ']'. Bu additionally change that rule to
detect partitioned loop devices instead of device-mapper.

(cherry picked from commit 68bbd408d7)

udev/udev-block-add-change

@@ -34,9 +34,9 @@ if [ "$MAJOR" -eq 7 -a ! -d /sys/$DEVPATH/loop ]; then
     xs_remove
     exit 0
 fi
-# ... and temporary device-mapper devices used during VM startup
-if [[ "$DM_NAME" = 'loop'* ] && \
-       [[ "`cat /sys/block/${DM_NAME%p*}/loop/backing_file`" = \
+# ... and temporary devices used during VM startup
+if [[ "$NAME" = 'loop'* ]] && \
+       [[ "`cat /sys/block/${NAME%p*}/loop/backing_file`" = \
             '/var/lib/qubes/'*'/volatile.img' ]]; then
     xs_remove
     exit 0