bdrv: Use "Error" for creating images

Add an Error ** parameter to BlockDriver.bdrv_create to allow more
specific error messages.

Signed-off-by: Max Reitz <mreitz@redhat.com>
diff --git a/block.c b/block.c
index 89098bc..f9b7033 100644
--- a/block.c
+++ b/block.c
@@ -401,7 +401,7 @@
     CreateCo *cco = opaque;
     assert(cco->drv);
 
-    cco->ret = cco->drv->bdrv_create(cco->filename, cco->options);
+    cco->ret = cco->drv->bdrv_create(cco->filename, cco->options, NULL);
 }
 
 int bdrv_create(BlockDriver *drv, const char* filename,