ANDROID: Incremental fs: Support xattrs

To make selinux work, add xattr support. This is a bit clunky -
it seems like it would be better for the log and pending read
functionality to be ioctls rather than this mixture of real
and virtual files.

Bug: 133435829
Change-Id: I56579fabe2ae7efb88f0344553948dc9573299aa
Signed-off-by: Paul Lawrence <paullawrence@google.com>