block: Make errp the last parameter of bdrv_img_create

Signed-off-by: Fam Zheng <famz@redhat.com>
Message-Id: <20170421122710.15373-6-famz@redhat.com>
Reviewed-by: Markus Armbruster <armbru@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Signed-off-by: Markus Armbruster <armbru@redhat.com>