qemu: hostdev: Fix the error on VM start with an mdev when IOMMU is off

qemu: hostdev: Fix the error on VM start with an mdev when IOMMU is off

Commit b4c2ac8d56 made a false assumption that IOMMU support necessary
for an mdev device to be assigned to a VM. Unlike direct PCI assignment,
IOMMU support is not needed for mediated devices, as the physical parent
device provides the isolation, therefore, simply checking for VFIO
presence is enough to successfully start a VM.

Luckily, this issue is not serious, since as of yet, libvirt mandates
mdevs to be pre-created prior to a domain's launch - if it is,
everything does work smoothly even with IOMMU disabled, because the
parent device will ensure the iommu groups we try to access exist.
However, if there are *no* IOMMU groups yet, thus no mdev exists yet, one
would see the following error:

"unsupported configuration: Mediated host device assignment requires VFIO
support"

The error msg above is simply wrong and doesn't even reflect the IOMMU
reality, so after applying this patch one would rather see the following
error in such case instead:

"device not found: mediated device '<UUID>' not found"

Signed-off-by: Erik Skultety <eskultet@redhat.com>