Prefer f2fs for virtual (private) storage volumes.

Since ext4 currently doesn't have the required kernel patches in place
on cuttlefish.

Bug: 150935323
Test: sm set-virtual-disk true
      sm partition disk:7,xyz private
      inspect mount output
Change-Id: Ief5bd9ace9d39bdfbae8d3857044a2143801f6be
1 file changed