Rework threading internals in XmlListModel to avoid global static

Rework threading internals in XmlListModel to avoid global static

QTBUG-20629 reports a crash on destruction of XmlListModel when
cleaning up the global static for the QDeclarativeXmlQuery object.

The fix restructures the internals to be like the threading structure
used for QDeclarativePixmapReader which doesn't use a global static.

Task-number: QTBUG-20629
Reviewed-by: Martin Jones
(cherry picked from commit 422f4e8ec53b917fad09a3e671fd93048dde72ed in
qt-qml-staging:master)

Change-Id: Ic26164947abab67ec9d8f1ae68c20961e7af8a2b
Reviewed-on: http://codereview.qt.nokia.com/2595
Reviewed-by: Qt Sanity Bot <qt_sanity_bot@ovi.com>
Reviewed-by: Bea Lam <bea.lam@nokia.com>