chardev: allow telnet gsource to switch gcontext

chardev: allow telnet gsource to switch gcontext

It was originally created by qio_channel_add_watch() so it's always
assigning the task to main context.  Now we use the new API called
qio_channel_add_watch_source() so that we get the GSource handle rather
than the tag ID.

Meanwhile, caching the gsource and TCPChardevTelnetInit (which holds the
handshake data) in SocketChardev.telnet_source so that we can also do
dynamic context switch when update read handlers.

Signed-off-by: Peter Xu <peterx@redhat.com>
Message-Id: <20180306053320.15401-5-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>