iommu-api: Extension to check for interrupt remapping

This patch allows IOMMU users to determine whether the
hardware and software support safe, isolated interrupt
remapping.  Not all Intel IOMMUs have the hardware, and the
software for AMD is not there yet.

Signed-off-by: Tom Lyon <pugs@cisco.com>
Signed-off-by: Joerg Roedel <joerg.roedel@amd.com>