block: Relative backing file for image creation

Relative backing filenames are always relative to the backed image's
directory; the same applies to image creation. Therefore, if the backing
file has to be opened for determining its size (in case the size has not
been explicitly specified) its filename should be interpreted relative
to the new image's base directory and not relative to qemu's working
directory.
Signed-off-by: NMax Reitz <mreitz@redhat.com>
Reviewed-by: NEric Blake <eblake@redhat.com>
Reviewed-by: NFam Zheng <famz@redhat.com>
Signed-off-by: NKevin Wolf <kwolf@redhat.com>

block.c

@@ -5659,16 +5659,26 @@ void bdrv_img_create(const char *filename, const char *fmt,
     if (size == -1) {
         if (backing_file) {
             BlockDriverState *bs;
+            char *full_backing = g_new0(char, PATH_MAX);
             int64_t size;
             int back_flags;
 
+            bdrv_get_full_backing_filename_from_filename(filename, backing_file,
+                                                         full_backing, PATH_MAX,
+                                                         &local_err);
+            if (local_err) {
+                g_free(full_backing);
+                goto out;
+            }
+
             /* backing files always opened read-only */
             back_flags =
                 flags & ~(BDRV_O_RDWR | BDRV_O_SNAPSHOT | BDRV_O_NO_BACKING);
 
             bs = NULL;
-            ret = bdrv_open(&bs, backing_file, NULL, NULL, back_flags,
+            ret = bdrv_open(&bs, full_backing, NULL, NULL, back_flags,
                             backing_drv, &local_err);
+            g_free(full_backing);
             if (ret < 0) {
                 goto out;
             }