[Libguestfs] Change to unstable mkfs-btrfs API

[Richard W.M. Jones]

I'm going to change the mkfs-btrfs API so its single and only
parameter is a list of devices (instead of a single device).
    
btrfs filesystems can span multiple devices, so this is necessary to
create such filesystems, eg:

  mkfs-btrfs ["/dev/vda1", "/dev/vdb1"]
  mount "/dev/vda1", "/"

would create a filesystem across two devices and mount it.

Note this changes the API, but this API has not yet been released in a
stable version of libguestfs, so we're allowed to do this.

Rich.

-- 
Richard Jones, Virtualization Group, Red Hat http://people.redhat.com/~rjones
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