QtCore: Fix remaining MSVC-64 warnings about integer truncation.

warning C4267: 'argument' : conversion from 'size_t' to ' int', possible
loss of data.

Change-Id: I8d1ca27d4d66a9ce0deb70766446e780308bba33
Reviewed-by: Mitch Curtis <mitch.curtis@digia.com>
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@digia.com>