Return storageprovider id in space root info

[butonic]

decomposedfs now returns the storageprovider id in the root info when listing spaces

turns

<d:multistatus xmlns:s="http://sabredav.org/ns" xmlns:d="DAV:" xmlns:oc="http://owncloud.org/ns">
    <d:response>
        <d:href>/remote.php/webdav/</d:href>
        <d:propstat>
            <d:prop>
                <oc:id>f7fbf8c8-139b-4376-b307-cf0a8c2d0d9c!f7fbf8c8-139b-4376-b307-cf0a8c2d0d9c</oc:id>
...

into

<d:multistatus xmlns:s="http://sabredav.org/ns" xmlns:d="DAV:" xmlns:oc="http://owncloud.org/ns">
    <d:response>
        <d:href>/remote.php/webdav/</d:href>
        <d:propstat>
            <d:prop>
                <oc:id>storage-users-1$f7fbf8c8-139b-4376-b307-cf0a8c2d0d9c!f7fbf8c8-139b-4376-b307-cf0a8c2d0d9c</oc:id>
...