Fixes LP Bug#885341 - Test failure in TestImageCacheManageXattr
    
    This bug was due to insufficient checking that not only was
    python-xattr installed, but also the device that the image
    cache dir lives in is actually able to have xattrs written
    to files (user_xattr in fstab). The fix adds a number of checks
    for xattr support on the device, both in the image cache configure()
    methods and in the test cases themselves (using @skip_if_disabled)
    
    Adds a functional test case for queue/prefetch of images into
    the image cache.
    
    Change-Id: I09b3d008fc00564031b0534155430ed0856e9e9c