virtio-9p: Implement Security model for mknod

Mapped mode stores extended attributes in the user space of the extended
attributes. Given that the user space extended attributes are available
to regular files only, special files are created as regular files on the
fileserver and appropriate mode bits are added to the extended attributes.
This method presents all special files and symlinks as regular files on the
fileserver while they are represented as special files on the guest mount.

On Host/Fileserver:
-rw-------. 1 virfsuid virtfsgid 0 2010-05-11 09:36 afifo
-rw-------. 1 virfsuid virtfsgid 0 2010-05-11 09:32 blkdev
-rw-------. 1 virfsuid virtfsgid 0 2010-05-11 09:33 chardev

On Guest/Client:
prw-r--r-- 1 guestuser guestuser 0 2010-05-11 12:36 afifo
brw-r--r-- 1 guestuser guestuser 0, 0 2010-05-11 12:32 blkdev
crw-r--r-- 1 guestuser guestuser 4, 5 2010-05-11 12:33 chardev

In the passthrough securit model, specifal files are directly created
on the fileserver. But the user credential

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