Change network speed reference for network_speed in virtual_guest
Change network speed reference for network_speed in virtual_guest

If a virtual guest only has private network interface, PrimaryNetworkComponent.MaxSpeed returns 0.
To support network_speed for the virtual guest with private network only, I changed network
 speed reference from PrimaryNetworkComponent.MaxSpeed to PrimaryBackendNetworkComponent.MaxSpeed.

softlayer/terraform-provider-softlayer,softlayer/terraform-provider-softlayer