block: Switch bdrv_write_zeroes() to byte interface

Rename to bdrv_pwrite_zeroes() to let the compiler ensure we
cater to the updated semantics.  Do the same for bdrv_co_write_zeroes().

Signed-off-by: Eric Blake <eblake@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
diff --git a/tests/qemu-iotests/034 b/tests/qemu-iotests/034
index c711cfc..1b28bda 100755
--- a/tests/qemu-iotests/034
+++ b/tests/qemu-iotests/034
@@ -1,6 +1,6 @@
 #!/bin/bash
 #
-# Test bdrv_write_zeroes with backing files
+# Test bdrv_pwrite_zeroes with backing files (see also 154)
 #
 # Copyright (C) 2012 Red Hat, Inc.
 #