chardev: convert the socket server to QIONetListener

Instead of creating a QIOChannelSocket directly for the chardev
server socket, use a QIONetListener. This provides the ability
to listen on multiple sockets at the same time, so enables
full support for IPv4/IPv6 dual stack.

Signed-off-by: Daniel P. Berrange <berrange@redhat.com>
Message-Id: <20171218135417.28301-2-berrange@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>