btrfs subvolume support

[asciiprod]

Currently when choosing btrfs, no subvolume is create as recommended
https://btrfs.wiki.kernel.org/index.php/SysadminGuide#Layout

It is also not possible to designate one partition as btrfs and create multiple subvolume with different mountpoints within.
The LVM code provides something similar, but is horrible and needs to do so much calculations whether there is still enough space. With btrfs we would just have to check that the toplevel volume (the device) itself has enough space for the image. We already do that more or less, by requiring that / is at least 1500MB