esx: Add support for virtual serial device network backing

Since version 4.1 ESX(i) can expose virtual serial devices over TCP.

Add support in the VMX handling code for this, add test cases to cover
it and add links to some documentation.

ESX supports two additional protocols: TELNETS and TLS. Add them to
the list of serial-over-TCP protocols.

tests/xml2vmxdata/xml2vmx-serial-network-server.vmx

@@ -0,0 +1,14 @@
+config.version = "8"
+virtualHW.version = "7"
+guestOS = "other"
+uuid.bios = "56 4d 9b ef ac d9 b4 e0-c8 f0 ae a8 b9 10 35 15"
+displayName = "serial-network"
+memsize = "4"
+numvcpus = "1"
+floppy0.present = "false"
+floppy1.present = "false"
+serial0.present = "true"
+serial0.fileType = "network"
+serial0.fileName = "ssl://0.0.0.0:42001"
+serial0.network.endPoint = "server"
+serial0.yieldOnMsrRead = "true"