Fix Qt-RangeLoopReference warnings

All cases use ranged-for loop to iterate over non-trivially-copyable
data (QVariant, QByteArray, QPointer).
In all cases it's better to take the values by const-ref, rather than
do actual copies.

Amends multiple patches, but all of them preceed Qt 6.8, so it's safe
to pick down to that branch.

Axivion-Id: qt_Add-ons_qtremoteobjects_dev_linux:SV78
Axivion-Id: qt_Add-ons_qtremoteobjects_dev_linux:SV88
Axivion-Id: qt_Add-ons_qtremoteobjects_dev_linux:SV89
Axivion-Id: qt_Add-ons_qtremoteobjects_dev_linux:SV90
Axivion-Id: qt_Add-ons_qtremoteobjects_dev_linux:SV91
Task-number: QTBUG-144772
Pick-to: 6.12 6.11 6.8
Change-Id: I24dea8db3dc562dd819e6add1bd4094f0caf73db
Reviewed-by: Tatiana Borisova <tatiana.borisova@qt.io>