Support use of xsi:type for polymorphic deserialization (FromXmlParser.Feature.AUTO_DETECT_XSI_TYPE)

[cowtowncoder]

(note: counterpart to generation-side #324 just implemented)

It is now possible to serializer POJOs like this one:

    @JsonRootName("Poly")
    @JsonTypeInfo(use = Id.SIMPLE_NAME, include = As.PROPERTY, property="xsi:type")
    static class PolyBean {
        public int value = 42;
    }

resulting in XML like:

<Poly xmlns:xsi='http://www.w3.org/2001/XMLSchema-instance' xsi:type='PolyBean'>"
  <value>42</value>
</Poly>

which is all fine; but we also need to support the part, deserialization, for seamless usability.
So let's do just that.