lxc-destroy: remove btrfs subvolumes

lxc-destroy: remove btrfs subvolumes

Doing this requires some btrfs functions from bdev to be used in
utils.c  Because utils.h is imported by lxc_init.c, I had to create
a new initutils.[ch] which are used by both lxc_init.c and utils.c
We could instead put the btrfs functions into utils.c, which would
be a shorter patch, but it really doesn't belong there.  So I went
the other way figuring there may be more such cases coming up of
fns in utils.c needing code from bdev.c which can't go into lxc_init.

Currently, if we detect a btrfs subvolume we just remove it.  The
st_dev on that dir is different, so we cannot detect if this is
bound in from another fs easily.  If we care, we should check
whether this is a mountpoint, this patch doesn't do that.

Signed-off-by: Serge Hallyn <serge.hallyn@ubuntu.com>
Acked-by: Stéphane Graber <stgraber@ubuntu.com>