This commit makes it possible to run test executables in a test
environment constructed of a virtual machine running the Linux kernel
with the virtual KMS driver enabled, and a mocked system environment
using meta-dbus-runner.py/python-dbusmock.
The qemu machine is configured to use 256M of memory, as the default
128M was not enough for the tests to pass.
Using qemu is also only made possible on x86_64; more changes are needed
for it to be runnable on aarch64, so add a warning if it was enabled on
any other architecture.
Part-of: <https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/2151>
