xpath_string(xml, xpath) - Returns the text contents of the first xml node that matches the xpath expression
Example:
  > SELECT xpath_string('<a><b>b</b><c>cc</c></a>','a/c') FROM src LIMIT 1;
  'cc'
  > SELECT xpath_string('<a><b>b1</b><b>b2</b></a>','a/b') FROM src LIMIT 1;
  'b1'
  > SELECT xpath_string('<a><b>b1</b><b>b2</b></a>','a/b[2]') FROM src LIMIT 1;
  'b2'
  > SELECT xpath_string('<a><b>b1</b><b>b2</b></a>','a') FROM src LIMIT 1;
  'b1b2'