Fire timer only when required.  Brings down wakeups by a big number.

# gpg: Signature made Fri Jul 17 14:41:40 2015 BST using RSA key ID 854083B6
# gpg: Good signature from "Amit Shah <amit@amitshah.net>"
# gpg:                 aka "Amit Shah <amit@kernel.org>"
# gpg:                 aka "Amit Shah <amitshah@gmx.net>"

* remotes/amit-virtio-rng/tags/vrng-2.4:
  virtio-rng: trigger timer only when guest requests for entropy

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>