afc: Fix accessing applications with HouseArrest

Only sideloaded applications (development versions for example) can use
the VendContainer method of accessing application documents.

Always use the VendDocuments method instead.

Note that we'll also re-root the Documents folder as the root of the app
container is inaccessible ("EPERM") to avoid users fiddling with it.

See https://github.com/libimobiledevice/libimobiledevice/issues/193#issuecomment-95280639

https://bugzilla.gnome.org/show_bug.cgi?id=763606