scripts/attach-static-vdis: Toggle nullglob to fix behavior on empty dir

[last-genius]

If STATE_DIR exists but is not empty, then globs for it in the form of STATE_DIR/* and STATE_DIR/*/vdi-uuid will be passed in literal form, causing errors like:

cat: /etc/xensource/static-vdis-test/*/vdi-uuid: No such file or directory

Toggle nullglob instead, which turns the whole glob string empty and does not enter the body of the loop iterating over the matches.

[changlei-li]

What else problem other than the cat error log? I think the error log is just no-harm

[last-genius]

What else problem other than the cat error log? I think the error log is just no-harm

exit code is 1 so the systemd service fails:

$ ./attach-static-vdis start
cat: /etc/xensource/static-vdis/*/vdi-uuid: No such file or directory
Attempting to attach VDI:
Attempt to attach VDI:  failed -- skipping (Error was: Traceback (most recent call last):
  File "/opt/xensource/bin/static-vdis", line 413, in <module>
    main()
  File "/opt/xensource/bin/static-vdis", line 404, in main
    disk_path = attach(sys.argv[2])
  File "/opt/xensource/bin/static-vdis", line 343, in attach
    raise Exception("Disk configuration not found")
Exception: Disk configuration not found)
$ echo $?
1
$ ./nullglob-attach-static-vdis start
Attempting to attach all statically-configured VDIs
$ echo $?
0