1. A system comprising: one or more input/output (I/O) devices; an I/O memory management unit (IOMMU) coupled to receive memory requests sourced by the I/O devices and configured to provide address translation for the memory requests; a virtual machine monitor (VMM) configured to manage one or more virtual machines on the system, wherein the VMM is configured to virtualize the IOMMU, providing one or more virtual IOMMUs for use by one or more virtual machines; and wherein a capabilities block defined for the IOMMU includes an indication of whether or not the IOMMU is configured to cache not-present translation table entries, and wherein the VMM is configured to intercept a read of the capabilities block from the virtual machine and is configured to respond to the read, and wherein the VMM is configured to place the indication, in the response data, in a state indicating that the IOMMU does cache not-present translation table entries.