[SolidHwTest] Extend FakeHW description with encrypted volume, add tests

As the StorageAccess is also folded into the Device of the encrypted
volume both encrypted and cleartext devices match the same filePath.

`storageAccessFromPath` could return any of the two according to its
documentation, as both provide StorageAccess and StorageVolume interfaces.
Extend the documentation to specify what is returned. Notably also
Devices which are not StorageVolumes can be returned, e.g. NetworkShares.

Add the corresponding test case.

See commit 18479ef3 ("skip non-FileSystem StorageVolumes in
storageAccessFromPath").