util: xml: Extract implementation of xml property -> enum parsing to a common helper

util: xml: Extract implementation of xml property -> enum parsing to a common helper

virXMLPropTristateBool/virXMLPropTristateSwitch/virXMLPropEnum can be
implemented using the same internal code. Extract it into a new function
called virXMLPropEnumInternal, which will also simplify adding versions
of these functions with a custom default value.

This way we'll be able to always initialize @result so that unused value
bugs can be prevented.

Signed-off-by: Peter Krempa <pkrempa@redhat.com>
Reviewed-by: Michal Privoznik <mprivozn@redhat.com>