virt-host-validate: check for IOMMU support

virt-host-validate: check for IOMMU support

This looks for existance of DMAR (Intel) and IVRS (AMD)
files under /sys/firmware/acpi/tables/, as a sign that
the platform has IOMMU present & enabled in the BIOS.

If these are present and /sys/kernel/iommu_groups does
not contain any entries this is taken as a sign that
the kernel has not enabled the IOMMU currently.

If no ACPI tables are found we can't distinguish between
disabled in BIOS and not present in the hardware, so we
have to give the user a generic hint.

Signed-off-by: Daniel P. Berrange <berrange@redhat.com>