qt: Do not assume qDBusRegisterMetaType return type

Summary:
qDBusRegisterMetaType returns:

• int in Qt 5
• QMetaType in Qt 6

This is a backport of core#584
Depends on D18140

Test Plan:
ninja all check-all

Not sure how to trigger that code path interactively. The change is probably trivial enough to assume it just works (we just inlined an unnecessary wrapper function).

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D18141