Fix determination of $oldroot from the config file. The old code had the '
in the wrong place and didn't account for whitespace between the = and
the rootfs.

Set $rootfs based on $oldroot instead of forcing it to be in
$lxc_path/$lxc_new/rootfs. This allows for btrfs snapshot to be made even if
$lxc_path isn't on the same filesystem. If $oldroot isn't a subvolume,
fall back to making a copy.
Signed-off-by: Dwight Engen <dwight.engen@oracle.com>