TLS handshake may create background GSource tasks, while we won't know
the correct GMainContext until the whole chardev (including frontend)
inited.  Let's postpone the initial TLS handshake until machine done.

For dynamically created tcp chardev, we don't postpone that by checking
the init_machine_done variable.

Signed-off-by: Daniel P. Berrange <berrange@redhat.com>
[peterx: add missing include line, do unit test]
Signed-off-by: Peter Xu <peterx@redhat.com>
Message-Id: <20180308140714.28906-1-peterx@redhat.com>
Acked-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Eric Blake <eblake@redhat.com>