add support for xml values

[djhaynes]

Many data sources (WMI, LDAP, SQL, etc) can store xml data. It is possible to
query one of these data sources where the resulting value is xml. OVAL does not
currently have a way of capturing and operating on these XML values.

Additionally, users have asked for the ability to examine the xml values
looking for the presence of elements, attributes, and their values.

Some additional thoughts:

• If an element has a value that is XML it is safe to assume that we will need
  the ability to query that XML . Queries could range from element/attribute
  values to counts to element names to existence checks. Does OVAL need to
  support xpath operations on XML data similar to how we support regular
  expressions on string data? Maybe we need an XML datatype and an xpath
  opperatoin?
• How do functions that are used in variables handle XML data?
• What operations should be allowed on XML data?