Add vmware workstation support.

Use a generic vmware model to support fusion and workstation. A generic
vmware provider will pick the appropriate provider. A warning message is
generated regarding missing plugin if vmware provider is requested and
neither fusion or workstation plugin is loaded.
This allows the specification of provider vmware and not bother duplicating two copies of provider: vmware_fusion and provider: vmware_workstation since the vmx settings are essentially the same.