chardev: tcp: postpone async connection setup

chardev: tcp: postpone async connection setup

This patch allows the socket chardev async connection be setup with
non-default gcontext.  We do it by postponing the setup to machine done,
since until then we can know which context we should run the async
operation on.

Reviewed-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Peter Xu <peterx@redhat.com>
Message-Id: <20180306053320.15401-8-peterx@redhat.com>
Acked-by: Stefan Hajnoczi <stefanha@redhat.com>
Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>