Fieldmapper toXML repairs and additions

Fieldmapper toXML repairs and additions

1. Be sure the append the current element to the document in progress

2. Use the class hint instead of the fully qualified class_name (e.g.
Fielmapper::actor::user) to avoid XML errors:

namespace error : Failed to parse QName 'Fieldmapper:'

3. Added support for additional options including "no_virt" which tells
the routine to skip all virtual fields and "skip_fields" to support
leaving specific fields out of the output.  The main use case for skip
fields is au => ['passwd'], but other examples might include large
fields like bre => ['marc'].

Signed-off-by: Bill Erickson <berick@esilibrary.com>