9pfs: rename virtio-9p-xattr{,-user}.{c,h} to 9p-xattr{,-user}.{c,h}

These three files are not virtio specific. Rename them to generic
names.

Fix comments and header inclusion in various files.

Signed-off-by: Wei Liu <wei.liu2@citrix.com>
Signed-off-by: Aneesh Kumar K.V <aneesh.kumar@linux.vnet.ibm.com>
diff --git a/hw/9pfs/9p-local.c b/hw/9pfs/9p-local.c
index 877ad86..ac553e0 100644
--- a/hw/9pfs/9p-local.c
+++ b/hw/9pfs/9p-local.c
@@ -12,7 +12,7 @@
  */
 
 #include "virtio-9p.h"
-#include "virtio-9p-xattr.h"
+#include "9p-xattr.h"
 #include "fsdev/qemu-fsdev.h"   /* local_ops */
 #include <arpa/inet.h>
 #include <pwd.h>