hw/9pfs: fix invalid union access by v9fs_co_fstat()

The individual FID types (P9_FID_NONE, P9_FID_FILE, P9_FID_DIR, P9_FID_XATTR)
share union V9fsFidOpenState with FID-type specific fields. Accessing any of
the union fields must comply with the FID-type to avoid undefined behaviour
or information disclosure.

Fix this in v9fs_lock() and v9fs_getlock() by checking if FID has a valid
file descriptor before calling v9fs_co_fstat().

Fixes: 10b468bdc533 ("virtio-9p: Implement TXATTRCREATE")
Link: https://lore.kernel.org/qemu-devel/4b33cd1aaa2551efda220a6f651e3660d27f4746.1781621428.git.qemu_oss@crudebyte.com
Signed-off-by: Christian Schoenebeck <qemu_oss@crudebyte.com>