cpr-transfer with caveats

[Steven Sistare <steven.sistare@oracle.com>]

Hi Peter, are you OK if we proceed with cpr-transfer as is, without the
precreate phase?  Here are the problems that motivated it:

* migration test wants to enable migration events on the dest.
   fix: enable on dest qemu using -global.  only for the test.

* migration test needs to fetch the dynamically assigned migration
     listen port number
   Fix: require unix domain socket for cpr-transfer, or a fixed port
   number. Document it.

* migration test hangs connecting to the qtest socket.
   fix: in the qtest code, defer connection.

Document that one cannot set migration caps or params on the dest
for cpr-transfer.

Document that for -incoming defer, mgmt must send the migrate command
to the src first (so dest reads cpr state and progresses to start the
monitor), then send the hotplug monitor commands to the dest.

Daniel, are you OK with that last bit?

- Steve