GET on specific non-config data for mountpoints fail

[olofhagsand]

For mount-points, cannot get specific state xpath that excludes config.
Example, suppose /top/my-mountpoint is a yang shema mount-point, and that the state tree yang-library is available directly under my-mointpoint, then
the following GET works:

    /top/my-mountpoint

with yang-library returned as part of the full mountpoint data. on the form:

   <top>
     <my-mointpoint>
        <config-data>....</config-data>
        <yang-library>...</yang-library>
     </my-mointpoint>
   </top>
```But the following returns nothing:

/top/my-mountpoint/yang-library

This may possibly have wider consequences also for non-mountpoints