hw/9pfs: Add new security model mapped-file.

This enable us to do passthrough equivalent security model on NFS directory.
NFS server mostly do root squashing and don't support xattr. Hence we cannot
use 'passthrough' or 'mapped' security model

Also added "mapped-xattr" security to indicate earlier "mapped" security model
Older name is still supported.

POSIX rules regarding ctime update on chmod are not followed by this security model.

Signed-off-by: Aneesh Kumar K.V <aneesh.kumar@linux.vnet.ibm.com>
