tests/qtest/readconfig: Test the docs/config/q35-*.cfg files

Test that we can successfully parse the docs/config/q35-emulated.cfg,
docs/config/q35-virtio-graphical.cfg and docs/config/q35-virtio-serial.cfg
config files (the "...-serial.cfg" file is a subset of the graphical
config file, so we skip that in quick mode).

These config files use two hard-coded image names which we have to
replace with unique temporary files to avoid race conditions in case
the tests are run in parallel. So after creating the temporary image
files, we also have to create a copy of the config file where we
replaced the hard-coded image names.

If KVM is not available, we also have to disable the "accel" lines.
Once everything is in place, we can start QEMU with the modified
config file and check that everything is available in QEMU.

Message-Id: <20230704071655.75381-4-thuth@redhat.com>
Signed-off-by: Thomas Huth <thuth@redhat.com>