virtio-9p: Use layered xattr approach

We would need this to make sure we handle the mapped
security model correctly for different xattr names.

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