udev: fix hiding devices from qvm-block

When device becomes non-attachable (for example because it gets mounted,
or used as part of LVM/RAID/whatever), it should be removed from
advertised available devices. The code for removing QubesDB entry was
buggy - the device is actually a directory in QubesDB, not a single
entry.

QubesOS/qubes-issues#1600

udev/udev-block-add-change

@@ -9,7 +9,7 @@ QDB_KEY="/qubes-block-devices/$NAME"
 
 xs_remove() {
     if [ "$QUBES_EXPOSED" == "1" ]; then
-        qubesdb-rm "$QDB_KEY"
+        qubesdb-rm "$QDB_KEY/"
         qubesdb-write /qubes-block-devices ''
     fi
     echo QUBES_EXPOSED=0