snapshot: fixed bdrv_get_full_backing_filename can not get correct full_backing_filename

This patch fixed the following bug:
https://bugzilla.redhat.com/show_bug.cgi?id=1084302 . path_combine can not calculate the correct full path name for backing_file. Such as:
create a snapshot chain:
sn2->sn1->$BASE_IMG
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.

---
 block.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

index 990a754..2519f68 100644
@@ -307,7 +307,7 @@ void bdrv_get_full_backing_filename(BlockDriverState *bs, char *dest, size_t sz)
     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);
+        realpath(bs->backing_file, dest);
     }
 }
 

1.8.3.1