valgrind support: Destroy mempool when memoryManager is destroyed
valgrind support: Destroy mempool when memoryManager is destroyed

Fixes "impossible situation!" abort in valgrind when running tests that
create and destroy a lot of engines.

Change-Id: I58cfd2380b0149d25ebecdb270be428ec6b8a64f
Reviewed-by: Lars Knoll <a812617a4072dcbeb80684eca0f369f0083af109@digia.com>

mgrunditz/qtdeclarative-2d,matthewvogt/qtdeclarative,qmlc/qtdeclarative,mgrunditz/qtdeclarative-2d,qmlc/qtdeclarative,matthewvogt/qtdeclarative,qmlc/qtdeclarative,matthewvogt/qtdeclarative,matthewvogt/qtdeclarative,mgrunditz/qtdeclarative-2d,qmlc/qtdeclarative,qmlc/qtdeclarative,mgrunditz/qtdeclarative-2d,matthewvogt/qtdeclarative,qmlc/qtdeclarative,matthewvogt/qtdeclarative,matthewvogt/qtdeclarative,qmlc/qtdeclarative,mgrunditz/qtdeclarative-2d,mgrunditz/qtdeclarative-2d,qmlc/qtdeclarative,mgrunditz/qtdeclarative-2d,matthewvogt/qtdeclarative