Stop using QVariant < operator

It will be removed in Qt 6 without porting strategy:
https://github.com/qt/qtbase/commit/f43cb31ba00a431c6d0a0b17750483a72ae03bb0

We know that that variants will be either ints (for `count`) or longs (for `size`),
so just convert them to longs (to avoid overflows) and compare those.