Prepare for 0.7.0 release.

Cargo.toml

@@ -1,6 +1,6 @@
 [package]
 name = "virtio-drivers"
-version = "0.6.0"
+version = "0.7.0"
 license = "MIT"
 authors = [
   "Jiajie Chen <noc@jiegec.ac.cn>",