bdev_create: don't default to btrfs if possible

Ideally it would be great to default to a btrfs subvolume for each new
container created.  However, this is not as we previously thought
without consequence.  'rsync --one-file-system' will not descend into
btrfs subvolumes.  This means that 'lxc-create -B _unset' will cause
different behavior for rsync -vax /var/lib/lxc based on whether that
fs is btrfs or not.

So don't do that.  If -B is not specified, use -B dir.

Signed-off-by: Serge Hallyn <serge.hallyn@ubuntu.com>
bdev: support -B best and -B lvm,dir

-B dev will check whether btrfs, zfs, or lvm can be used,
in that order, and fall back to dir.

-B lvm,btrfs will try lvm first, then btrfs, then fail.

Signed-off-by: Serge Hallyn <serge.hallyn@ubuntu.com>
document new lxc-create btrfs behavior

Signed-off-by: Serge Hallyn <serge.hallyn@ubuntu.com>