tests/9pfs: wipe local 9pfs test directory

tests/9pfs: wipe local 9pfs test directory

Before running the first 9pfs test case, make sure the test directory
for running the 9pfs 'local' tests on is entirely empty. For that
reason simply delete the test directory (if any) before (re)creating
it on test suite startup.

Note: The preferable precise behaviour would be the test directory
only being wiped once *before* a test suite run. Right now the test
directory is also wiped at the *end* of a test suite run because
libqos is calling the virtio_9p_register_nodes() callback for some
reason also when a test suite completed. This is suboptimal as
developers cannot immediately see what files and directories the
9pfs local tests created precisely after the test suite completed.
But fortunately the test directory is not wiped if some test failed.
So it is probably not worth it drilling another hole into libqos
for this issue.

Signed-off-by: Christian Schoenebeck <qemu_oss@crudebyte.com>
Message-Id: <b30776ea3289dc40dabc7d0063d825d21d9a65bf.1602182956.git.qemu_oss@crudebyte.com>
Signed-off-by: Christian Schoenebeck <qemu_oss@crudebyte.com>