udev-block-add-change: don't exclude already attached devs

For example, attaching a device-mapper device to another VM increases
its "dmsetup info" open count in is_used(), which triggered its removal
from qvm-block.

(cherry picked from commit 6b32378158)

udev/udev-block-add-change

@@ -11,6 +11,10 @@ MODE=w
 QDB_KEY="/qubes-block-devices/$NAME"
 
 xs_remove() {
+    if is_attached /sys$DEVPATH; then
+        return 0
+    fi
+
     if [ "$QUBES_EXPOSED" == "1" ]; then
         qubesdb-rm "$QDB_KEY/"
         qubesdb-write /qubes-block-devices ''