Ability to create sparse voumes

[mateuszkwiatkowski]

Hello,
I'd like to be able to create sparse volumes using this library. To do that with zfs command one need to specify -s flag so a function like this would work (it's just a copy of CreateVolume with additional flag in args[2]):

// CreateVolume creates a new ZFS volume with the specified name, size, and
// properties.
// A full list of available ZFS properties may be found here:
// https://www.freebsd.org/cgi/man.cgi?zfs(8).
func CreateSparseVolume(name string, size uint64, properties map[string]string) (*Dataset, error) {
	args := make([]string, 4, 5)
	args[0] = "create"
	args[1] = "-p"
	args[2] = "-sV"
	args[3] = strconv.FormatUint(size, 10)
	if properties != nil {
		args = append(args, propsSlice(properties)...)
	}
	args = append(args, name)
	_, err := zfs(args...)
	if err != nil {
		return nil, err
	}
	return GetDataset(name)
}