virtio-9p: Security model for create/open2

In the mapped security model, VirtFS server intercepts and maps
the file object create and get/set attribute requests. Files on the fileserver
will be created with VirtFS servers (QEMU) user credentials and the
client-users credentials are stored in extended attributes. On the request
to get attributes, server extracts the client-users credentials
from extended attributes and sends them to the client.

On Host/Fileserver:
-rw-------. 2 virfsuid virtfsgid 0 2010-05-11 09:19 afile

On Guest/Client:
-rw-r--r-- 2 guestuser guestuser 0 2010-05-11 12:19 afile

Signed-off-by: Venkateswararao Jujjuri <jvrao@linux.vnet.ibm.com>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>
