bdrv: Use "Error" for opening images

Add an Error ** parameter to BlockDriver.bdrv_open and
BlockDriver.bdrv_file_open to allow more specific error messages.

Signed-off-by: Max Reitz <mreitz@redhat.com>
diff --git a/block/vmdk.c b/block/vmdk.c
index fb5b529..d60bf0c 100644
--- a/block/vmdk.c
+++ b/block/vmdk.c
@@ -806,7 +806,8 @@
     return ret;
 }
 
-static int vmdk_open(BlockDriverState *bs, QDict *options, int flags)
+static int vmdk_open(BlockDriverState *bs, QDict *options, int flags,
+                     Error **errp)
 {
     int ret;
     BDRVVmdkState *s = bs->opaque;