char: fix repeated registration of tcp chardev I/O handlers

In previous commit:

  commit f2001a7e
  Author: Daniel P. Berrange <berrange@redhat.com>
  Date:   Tue Jan 19 11:14:30 2016 +0000

    char: don't assume telnet initialization will not block

The code which writes the telnet initialization sequence moved
to an event loop callback. If the TCP chardev is opened as a
server in blocking mode (ie -serial telnet:0.0.0.0:3000,server,wait)
this results in a state where the TCP chardev is connected, but not
yet ready to send/recv data when virtual hardware is created.

When the virtual hardware initialization registers its chardev
callbacks, it triggers tcp_chr_update_read_handler, which will
add I/O watches to the connection.

When the telnet initialization finally runs, it will then call
tcp_chr_connect to finish the connection setup. This will in
turn add I/O watches to the connection too.

There are now two sets of I/O watches registered on the same
connection. This ultimately causes data loss on the connection,
for example, when typing into the telnet console only every
second byte is echoed back to the client.

The same flaw can affect channels running with TLS encryption
too, since they also have delayed connection setup completion.

The fix is to update tcp_chr_update_read_handler so that it
avoids registering watches if the connection is not fully
setup yet.

Signed-off-by: Daniel P. Berrange <berrange@redhat.com>
Message-Id: <1454939707-10869-1-git-send-email-berrange@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>