PWE, VT-d, DMAR ACPI

I'm trying out Parallels Workstation Extreme because I need VGA passthrough, and under File -> Preferences -> Intel VT-d I see a message saying that Initialization of Intel VT-d failed because the DMAR ACPI table could not be found.

But dmesg shows:
Intel-IOMMU: enabled
[...]
ACPI: RSDP 00000000000f9f70 00024 (v02 ACPIAM)
ACPI: XSDT 00000000af790100 00064 (v01 042413 XSDT1438 20130424 MSFT 00000097)
[...]
ACPI: DMAR 00000000af79e0d0 00128 (v01 AMI OEMDMAR 00000001 MSFT 00000097)

When prl_hypervisor is loading, I it shows this:
prl_hypervisor: module license 'unspecified' taints kernel.
Disabling lock debugging due to kernel taint
/drv/ HypVtx.c:186 CPU is Intel
/drv/ HypLowCache.c:193 Low cache initialized (0 kB for 0 VMs on 48452 MB)
/drv/ HypApic.c:247 Host APIC base_msr 0xFEE00800 ver 0x15
/drv/ HypVtd.c:3964 [vtdInit]
/drv/ HypAcpi.c:160 map: addr = 0x9d000, size = 1024
/drv/ HypAcpi.c:111 [acpiScanForRsdp] RSDP NOT found!
ioremap error for 0xaf790000-0xaf791000, requested 0x10, got 0x0
/drv/ HypAcpi.c:241 [acpiMapTable] Can't map physical address 0x00000000af790100(36)!
/drv/ HypAcpi.c:351 [_acpiFindTableBySig] Can't map RSDT/XSDT table!
/drv/ HypVtd.c:3709 [vtdProcessDmarTable] Can't find DMAR table!
/drv/ HypVtd.c:4005 [vtdInit] No VT-d hardware

So the kernel is showing that the required ACPI tables are there, but prl_hypervisor is claiming they aren't. What is going on here?

I am using EL6 kernel 2.6.32-358.6.1.el6.x86_64, am using the following boot parameters:
selinux=0 intel_iommu=on iomem=relaxed

The CPU is a Xeon X5650 and the hardware supports VT-d (I can successfully pass PCI devices through to VMs using Xen and KVM).

Googling the problem has yielded several similar posts, but no resolution was ever provided.

I also tried with intel_iommu=off intremap=off kernel boot parameters and it made no difference.