char: don't assume telnet initialization will not block

The current code for doing telnet initialization is writing to
a socket without checking the return status. While it is highly
unlikely to be a problem when writing to a bare socket, as the
buffers are large enough to prevent blocking, this cannot be
assumed safe with TLS sockets. So write the telnet initialization
code into a memory buffer and then use an I/O watch to fully
send the data.

Signed-off-by: Daniel P. Berrange <berrange@redhat.com>
Message-Id: <1453202071-10289-4-git-send-email-berrange@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>