9pfs: convert 'len/copied_len' field in V9fsXattr to the type of uint64_t

The 'len' in V9fsXattr comes from the 'size' argument in setxattr()
function in guest. The setxattr() function's declaration is this:

int setxattr(const char *path, const char *name,
             const void *value, size_t size, int flags);

and 'size' is treated as u64 in linux kernel client code:

int p9_client_xattrcreate(struct p9_fid *fid, const char *name,
                          u64 attr_size, int flags)

So the 'len' should have an type of 'uint64_t'.
The 'copied_len' in V9fsXattr is used to account for copied bytes, it
should also have an type of 'uint64_t'.

Suggested-by: Greg Kurz <groug@kaod.org>
Signed-off-by: Li Qiang <liqiang6-s@360.cn>
Reviewed-by: Greg Kurz <groug@kaod.org>
Signed-off-by: Greg Kurz <groug@kaod.org>

hw/9pfs/9p.h

@@ -159,8 +159,8 @@ typedef struct V9fsConf
 
 typedef struct V9fsXattr
 {
-    int64_t copied_len;
-    int64_t len;
+    uint64_t copied_len;
+    uint64_t len;
     void *value;
     V9fsString name;
     int flags;