Virtualization support for a 64-bit guest OS is not dependent on Intel's VT-d technology. VT-x alone is sufficient for a 64-bit guest.
Intel's VT-d is used for I/O passthrough (also referred to as PCI passthrough).
From Wikipedia - x86 virtualization:
I/O MMU virtualization (AMD-Vi and Intel VT-d):
An input/output memory management unit (IOMMU) allows guest virtual
machines to directly use peripheral devices, such as Ethernet,
accelerated graphics cards, and hard-drive controllers, through DMA
and interrupt remapping. This is sometimes called PCI passthrough
See also from David Ott's article on software.intel.com:
VT-d, at the time of this writing, includes four key capabilities
I/O device assignment. This feature allows an administrator to assign I/O devices to VMs in any desired configuration.
DMA remapping. Supports address translations for device DMA data transfers.
Interrupt remapping. Provides VM routing and isolation of device interrupts.
Reliability features. Reports and records system software DMA and interrupt erros that may otherwise corrupt memory of impact VM
Note that VT-d is not dependent on VT-x. That is, a VT-x enabled
system can operate without VT-d, or without VT-d enabled or
configured. You simply miss the benefits of the feature. Many people
have asked about this point.
And here's one more reference that specifically mentions that VT-d isn't relevant when it comes to 64-bit guest capability in a VM. From Virtualbox Forum's Canonical post "I have a 64bit host, but can't install 64bit guests":
You usually need to enable VT-x/AMD-v in the host PC BIOS. You need to
check with your PC manual or support forum to find out how to boot
into the BIOS screen. This is probably not something we here at the
VirtualBox forums can help you with. Once you get there you need to
look for something buried in a menu, perhaps in the security category.
The option may be called something like "Enable Virtualization
Technology". If you see "Virtual Directed I/O" (VT-d/AMD-Vi) then that
is a different thing. Remember to power cycle your host PC after
making and saving the BIOS changes - in this case a full restart from
power off is required, just rebooting or resuming from a hibernated
state may not do the job.