[3.12] community/qemu: run pre-install as pre-upgrade hook so users are created during upgrade

We introduced a new user in qemu previously with 2f32bfc7, but that user is only ever created when qemu is installed from scratch and not when upgrading an exisiting qemu installation. This broke upgrading from 3.11 -> 3.12 where we expect the qemu user to be available in 3.12 but it wasn't unless you removed and installed qemu again. Also redirect the stderr of adduser to /dev/null so users don't see "adduser: user 'qemu' in use" during upgrades.

ref #11618 (closed)

(cherry picked from commit e100ed3c)