[Spaces] New Spaces type for "Share Jail"

Currently the "Share Jail" has the type "virtual". According to @butonic , there might be more "virtual" Spaces in the future. That's why clients will rely on the UUID (https://github.com/owncloud/client/issues/10196 to identify the Share Jail, https://github.com/owncloud/ios-app/issues/1164), but this feels fragile too. Desktop and iOS app will use the virtual Share Jail for quite some while, because they can't deal with single-file mountpoints. My proposal would be a new Space type, only for the "Share Jail".

/cc @TheOneRing @felix-schwarz @kulmann @micbar