[ACM-security] Some fixes to tools.

[ACM-security] Some fixes to tools.

- Allow multiple ChineseWallTypes in a VM labels
- check for surfacing exceptions in the python code
- check for array sizes in the XML DOM in python
- properly display the labels when doing 'xm list --label' in xm's
non-Xen-API mode
- rely on the security checking hooks in xend to check access to the
block interface rather than doing this in xm.

Signed-off-by: Stefan Berger <stefanb@us.ibm.com>