add main_dispatcher, a message passing mechanism for sending messages to
the main thread. The main thread is the thread that implements
SpiceCoreInterface, which is assumed to be a single thread.

Similar to the async operation of red_worker, a socket pair is created
and used to pass messages. The messages are a fixed size to ease
parsing. A single message is defined to pass a channel_event.

RHBZ: 746950
FDBZ: 41858

This patch is 0.8 branch only, for the master branch there should be a
better approach to share code with red_dispatcher and ready the way for
later adding more threads.

fix wrongly applied master patch. With this it is identical to the
change in master commit
 b37182426923124470bf3701f4cff2a6bb345b8d

When the server is a migration target and spice_server_migrate_connect
is called before SPICE_MSGC_MIGRATE_END has been received, we start
the mig_timer. We handle the migrate_connect only when receiving SPICE_MSGC_MIGRATE_END.
If the mig_timer expires before that, we dismiss the request, and should call the
migrate_connect completion callback. Since reds->mig_inprogress
wasn't set appropriately, it wasn't called.

We will add a qemu call to spice_server_migrate_start when migration starts.
For now, it does nothing, but we may need this notification in the future.

(1) not sending anything to the client till we recieve SPICE_MSGC_MIGRATE_END
(2) start a new migration (handle client_migrate_info) only after SPICE_MSGC_MIGRATE_END
    from the previous migration has been received
(3) use the correct ticket

Note: we assume the same channles are linked before and ater migration. i.e.,
SPICE_MSGC_MAIN_ATTACH_CHANNELS is not sent from the client.