qtest/virtio-9p-test.c: create/remove temp dirs after each test

qtest/virtio-9p-test.c: create/remove temp dirs after each test

The local 9p driver in virtio-9p-test.c its temporary dir right at the
start of qos-test (via virtio_9p_create_local_test_dir()) and only
deletes it after qos-test is finished (via
virtio_9p_remove_local_test_dir()).

This means that any qos-test machine that ends up running virtio-9p-test
local tests more than once will end up re-using the same temp dir. This
is what's happening in [1] after we introduced the riscv machine nodes:
if we enable slow tests with the '-m slow' flag using
qemu-system-riscv64, this is what happens:

- a temp dir is created;

- virtio-9p-device tests will run virtio-9p-test successfully;

- virtio-9p-pci tests will run virtio-9p-test, and fail right at the
  first slow test at fs_create_dir() because the "01" file was already
created by fs_create_dir() test when running with the virtio-9p-device.

The root cause is that we're creating a single temporary dir, via the
construct/destruct callbacks, and this temp dir is kept for the entire
qos-test run.

We can change each test to clean after themselves. This approach would
make the 'create' tests obsolete since we would need to create and
delete dirs/files/symlinks for the cleanup, turning them into the
'unlinkat' tests that comes right after.

We chose a different approach that handles the root cause: do not use
constructor/destructor to create the temp dir. Create one temp dir for
each test, and remove it after the test is complete. This is the
approach taken for other qtests like vhost-user-test.c where each test
requires a setup() and a subsequent cleanup(), all of those instantiated
in the .before callback.

[1] https://mail.gnu.org/archive/html/qemu-devel/2024-03/msg05807.html

Reported-by: Thomas Huth <thuth@redhat.com>
Signed-off-by: Daniel Henrique Barboza <dbarboza@ventanamicro.com>
Message-Id: <20240327142011.805728-2-dbarboza@ventanamicro.com>
Reviewed-by: Greg Kurz <groug@kaod.org>
Reviewed-by: Christian Schoenebeck <qemu_oss@crudebyte.com>
Tested-by: Thomas Huth <thuth@redhat.com>
Signed-off-by: Christian Schoenebeck <qemu_oss@crudebyte.com>