udev-node: do not create symlink to a non-existing device node

udev-node: do not create symlink to a non-existing device node

Previously, the stack directory contains empty regular files named with
device ID, and we create sd_device object from the device name.
Hence, we implicitly checked the existence of the device node.

However, now the files in the stack directory are symlink, and we
retrieve the path to the device node and its priority from the symlink.
Hence, the existence of the device node is not checked.
Let's check if the device node is still exist.