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.

Also fix 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.

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