tests/qtest/libqtest: Replace QTEST_TRACE with QTEST_QEMU_ARGS

tests/qtest/libqtest: Replace QTEST_TRACE with QTEST_QEMU_ARGS

The QTEST_TRACE environment variable allows for any QEMU command line
option to be passed if used like so:

export QTEST_TRACE="-trace tracepoint -more -opts -here"

Formalize that usage by accepting a new QTEST_QEMU_ARGS
variable. Since the QTEST_TRACE now becomes redundant, remove its
usage.

Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Link: https://lore.kernel.org/qemu-devel/20260429003130.15164-2-farosas@suse.de
[end all fmt strings with space]
Signed-off-by: Fabiano Rosas <farosas@suse.de>