add zfs support to lxc-create and lxc-destroy

This is based on patch from Papp Tamas (thanks).  It also does some
reorganizing of lxc-create to commonize some of the backingstore handling.

I played with it using:

	sudo lvcreate -L 100G -n zfs vg0
	sudo zpool create lxc /dev/vg0/zfs
	sudo lxc-create -B zfs --zfsroot lxc -t ubuntu -n dir2

or you could

	qemu-img create zfs.img 100G
	sudo qemu-nbd -c /dev/nbd0 zfs.img
	sudo zpool create lxc /dev/nbd0
	sudo lxc-create -B zfs --zfsroot lxc -t ubuntu -n dir2

I'll write the bdev.c handler and hook up lxc-clone next.

This also fixses a bug in the sed expression to extract the rootfs from
container config, which prepended an extra '/' to the rootdev.  (That
caused the zfs list entry not to match at destroy)

Signed-off-by: Serge Hallyn <serge.hallyn@ubuntu.com>
Cc: Papp Tamas <tompos@martos.bme.hu>
Acked-by: Stéphane Graber <stgraber@ubuntu.com>