sfc: Improve test for IOMMU in use
sfc: Improve test for IOMMU in use

The device::iommu_group field may be set even if no IOMMU is in use.
iommu_present() is still a better indicator, although it doesn't tell
us whether *our* device is affected.

Reported-by: Alex Williamson <7469de9b95ba379e2656fa9677689657c47a7690@redhat.com>
Signed-off-by: Ben Hutchings <0672176982fbc629cc374f31a7a45ae1671abf90@solarflare.com>

KristFoundation/Programs,KristFoundation/Programs,KristFoundation/Programs,KristFoundation/Programs,KristFoundation/Programs,KristFoundation/Programs