[libvirt] Extend virConfParser to allow parsing VMware VMX config files

Hi, As part of the VMware ESX driver I need to parse VMware VMX config files. This files have basically the same syntax as Xen config files, but the key part of the key=value format is allowed to contain ':' and '.'. The attached patch adds a flag to the virConfParser to enabled a VMX mode for the parser to allow ':' and '.' in the key, as suggested by Daniel P. Berrange on IRC. Regards Matthias