If GNOME_VFS_XFER_REMOVESOURCE is passed to gnome_vfs_async_xfer, the move
sometimes fails if the source and the target are on different file systems,
since it calls gnome_vfs_move_uri, which checks for check_same_fs_in_uri in
gnome-vfs-cancellable-ops.c:gnome_vfs_move_uri_cancellable.
gnome_vfs_async_xfer should rather copy the files and clean up the source
afterwards if the files are on different FSes.
gnome_vfs_xfer_uri_internal seems to do the right thing if
GNOME_VFS_XFER_USE_UNIQUE_NAMES is TRUE and gnome_vfs_check_same_fs_uris returns
GNOME_VFS_OK which - from what I can see in file-method.c - may not always be
the case. Maybe we have to
a) remove the _USE_UNIQUE_NAMES branch and always do the same_fs voodoo
b) also check for GNOME_VFS_ERROR_INVALID_URI when deciding whether to break in
the same_fs loop.