Several backing store improvements

allow copy clones from other bdevs

for lvm and zfs, as we don't yet support passing options, only default
VG of 'lxc' and default zfsroot of 'tank' are supported when converting
another backing store type.

refuse deletion of container which has lvm or zfs snapshots.
	Note that since a zfs clone must be made from a zfs snapshot,
	which is made from the original zfs fs, even after we
	lxc-destroy the snapshotted container we still must manually
	remove the snapshot.  This can be handled automatically, by
	looking for snapshots where c1 is the original, c2 is the clone,
	tank/c2 no longer exists, but tank/c1@c2 does.  We can then
	remove tank/c1@c2 and feel free to remove tank/c1.  This patch
	does NOT do that yet.

Make sure not to return when we're a forked child.

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