chardev-socket: do not blindly reset handlers when switching GMainContext

chardev-socket: do not blindly reset handlers when switching GMainContext

If the socket is connecting or connected, tcp_chr_update_read_handler will
be called but it should not set the NetListener's callbacks again.
Otherwise, tcp_chr_accept is invoked while the socket is in connected
state and you get an assertion failure.

Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>
Reviewed-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>