tests/9pfs: fix test dir for parallel tests

tests/9pfs: fix test dir for parallel tests

Use mkdtemp() to generate a unique directory for the 9p 'local' tests.

This fixes occasional 9p test failures when running 'make check -jN' if
QEMU was compiled for multiple target architectures, because the individual
architecture's test suites would run in parallel and interfere with each
other's data as the test directory was previously hard coded and hence the
same directory was used by all of them simultaniously.

This also requires a change how the test directory is created and deleted:
As the test path is now randomized and virtio_9p_register_nodes() being
called in a somewhat undeterministic way, that's no longer an appropriate
place to create and remove the test directory. Use a constructor and
destructor function for creating and removing the test directory instead.
Unfortunately libqos currently does not support setup/teardown callbacks
to handle this more cleanly.

The constructor functions needs to be in virtio-9p-test.c, not in
virtio-9p.c, because in the latter location it would cause all apps that
link to libqos (i.e. entirely unrelated test suites) to create a 9pfs
test directory as well, which would even break other test suites.

Signed-off-by: Christian Schoenebeck <qemu_oss@crudebyte.com>
Reviewed-by: Greg Kurz <groug@kaod.org>
Tested-by: Greg Kurz <groug@kaod.org>
Message-Id: <7746f42d8f557593898d3d9d8e57c46e872dfb4f.1604243521.git.qemu_oss@crudebyte.com>
Signed-off-by: Christian Schoenebeck <qemu_oss@crudebyte.com>