snapshot: fixed bdrv_get_full_backing_filename can not get correct full_backing_filename

This patch fixed bdrv_get_full_backing_filename can not calculate the correct full path name for backing_file via path_combine.

Such as:
create a snapshot chain:
$BASE_IMG<-sn1<-sn2
backing file is : /home/wookpecker/img.qcow2
sn1 : /home/woodpecker/tmp/sn1
sn2 : /home/woodpecker/tmp/sn2
when create sn2, path_combine can not got a correct path for $BASE_IMG.

In this patch, will check the backing_filename is a symlink or not firstly, then return the full(absolute) path via realpath.

Signed-off-by Jun Li <[email protected]>

---
This is v4 of the patch. This version is the same with v3. Only difference is the commit description. What does this patch has fixed, please ref following bug(it gives the detailed description):
https://bugzilla.redhat.com/show_bug.cgi?id=1084302

---
  block.c | 18 +++++++++++++++++-
  1 file changed, 17 insertions(+), 1 deletion(-)

block.c +17 -1

index c90c71a..d163a8c 100644
@@ -304,10 +304,26 @@ void path_combine(char *dest, int dest_size,
 
  void bdrv_get_full_backing_filename(BlockDriverState *bs, char *dest, size_t sz)
  {
+    struct stat sb;
+    char *linkname;
+
      if (bs->backing_file[0] == '\0' || path_has_protocol(bs->backing_file)) {
          pstrcpy(dest, sz, bs->backing_file);
      } else {
-        path_combine(dest, sz, bs->filename, bs->backing_file);
+        if (lstat(bs->backing_file, &sb) == -1) {
+            perror("lstat");
+            exit(EXIT_FAILURE);
+        }
+
+        /* Check linkname is a link or not */
+        if (S_ISLNK(sb.st_mode)) {
+            linkname = malloc(sb.st_size + 1);
+            readlink(bs->backing_file, linkname, sb.st_size + 1);
+            linkname[sb.st_size] = '\0';
+            realpath(linkname, dest);
+        } else {
+            realpath(bs->backing_file, dest);
+        }
      }
  }