Merge remote-tracking branch 'remotes/amit/tags/vser-for-2.3-3' into staging

virtio-serial api: guest_writable callback for users

# gpg: Signature made Thu Mar 19 12:06:55 2015 GMT using RSA key ID 854083B6
# gpg: Good signature from "Amit Shah <[email protected]>"
# gpg:                 aka "Amit Shah <[email protected]>"
# gpg:                 aka "Amit Shah <[email protected]>"

* remotes/amit/tags/vser-for-2.3-3:
  virtio: serial: expose a 'guest_writable' callback for users

Signed-off-by: Peter Maydell <[email protected]>