ZFS's configuration needs to be updated whenever the physical path for a
device changes, but not when a new device is introduced. This is because
new devices necessarily cause config updates, but only if they are actually
accepted into the pool.
- Split vdev_geom_set_physpath out of vdev_geom_attrchanged
- When setting the vdev's physical path, only request a config
update if the physical path has changed. Don't request it when opening a device for the first time, because the config sync will happen anyway upstack.
- Split g_dev_set_physpath and g_dev_set_media out of