Fix the console ring reader thread

When data appeared on the console ring the thread would spin.

Signed-off-by: David Scott <dave.scott@eu.citrix.com>
Use a simple handshake before starting the entropy stream

The protocol is described in doc/protocol.md. The protocol is
intended to make sure that the connection has been made properly--
it would be a disaster if the guest read data from somewhere
else and assumed it was entropy.

Fixes #3

Signed-off-by: David Scott <dave.scott@eu.citrix.com>