virtio-9p: Add SM_NONE security model

This is equivalent to SM_PASSTHROUGH security model.
The only exception is, failure of privilige operation like chown
are ignored. This makes a passthrough like security model usable
for people who runs kvm as non root

Signed-off-by: Aneesh Kumar K.V <aneesh.kumar@linux.vnet.ibm.com>
diff --git a/vl.c b/vl.c
index fd491ba..df11ab3 100644
--- a/vl.c
+++ b/vl.c
@@ -2320,7 +2320,7 @@
                         qemu_opt_get(opts, "path") == NULL ||
                         qemu_opt_get(opts, "security_model") == NULL) {
                     fprintf(stderr, "Usage: -virtfs fstype,path=/share_path/,"
-                            "security_model=[mapped|passthrough],"
+                            "security_model=[mapped|passthrough|none],"
                             "mnt_tag=tag.\n");
                     exit(1);
                 }