blockdev: convert qemu-nbd server to QIONetListener

Instead of creating a QIOChannelSocket directly for the NBD
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. This also means we can
honour multiple FDs received during socket activation.

Reviewed-by: Eric Blake <eblake@redhat.com>
Signed-off-by: Daniel P. Berrange <berrange@redhat.com>
Message-Id: <20171218101643.20360-3-berrange@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>