QT_INSTALL_QML prefix points to sysroot when cross compiling, so those
paths cannot be used for the QML registration. Instead use the path
available from QLibraryInfo at runtime.

Change-Id: I6556478558a3dda61e432aeef5f13667b9da626d
Task-number: QTRD-3400
Reviewed-by: Mitch Curtis <mitch.curtis@theqtcompany.com>
Reviewed-by: Gatis Paeglis <gatis.paeglis@theqtcompany.com>