Disable the usage of the XHCI bus for USB on the vmware-iso builder.

Some platforms with incomplete XHCI implementations (i.e. FreeBSD)
will poll the bus despite there being no usb devices available.
This disables XHCI by default and documents how to enable it using
the vmx_data option.

This closes issue #5961.

builder/vmware/iso/step_create_vmx.go

@@ -716,7 +716,6 @@ tools.upgrade.policy = "upgradeAtPowerCycle"
 // USB
 usb.pciSlotNumber = "32"
 usb.present = "{{ .Usb_Present }}"
-usb_xhci.present = "TRUE"
 
 // Serial
 serial0.present = "{{ .Serial_Present }}"

website/source/docs/builders/vmware-iso.html.md

@@ -357,7 +357,9 @@ builder.
     By default the upload path is set to `{{.Flavor}}.iso`. This setting is not
     used when `remote_type` is "esx5".
 
--   `usb` (boolean) - Enable VMware's USB bus for the VM.
+-   `usb` (boolean) - Enable VMware's USB bus for the guest VM. To enable usage
+    of the XHCI bus for USB 3 (5 Gbit/s), one can use the `vmx_data` option to
+    enable it by specifying "true" for the `usb_xhci.present` property.
 
 -   `version` (string) - The [vmx hardware
     version](http://kb.vmware.com/selfservice/microsites/search.do?language=en_US&cmd=displayKC&externalId=1003746)