[v6] Prefer xz vdb over rafs

[prabhu]

vdb6 is switching to a sqlite db from the current file-based one. xz compression seems to be performing well with sqlite compared to nydus rafs - 174MB vs 496MB with the latest.

/mnt/work  ls -lh rafs_out                                                                                                                                                                              total 497M
-rw-r--r-- 1 prabhu prabhu 496M Mar 19 14:16 data.rafs
-rw-r--r-- 1 prabhu prabhu 1.2M Mar 19 14:16 meta.rafs

/mnt/work  ls -lh vdb                                                                                                                                                                                         total 13G
-rw-r--r-- 1 prabhu prabhu 434M Mar 19 14:15 data.index.vdb6
-rw-r--r-- 1 prabhu prabhu  34M Mar 19 14:15 data.index.vdb6.tar.xz
-rw-r--r-- 1 prabhu prabhu  13G Mar 19 14:15 data.vdb6
-rw-r--r-- 1 prabhu prabhu 140M Mar 19 14:15 data.vdb6.tar.xz