virpcimock: Create symlink in /sys/kernel/iommu_groups/N/devices dir

virpcimock: Create symlink in /sys/kernel/iommu_groups/N/devices dir

So far, we don't need to create anything under
/sys/kernel/iommu_groups/N/devices directory (which is symlinked
from /sys/bus/pci/devices/DDDD:BB:DD.F/iommu_group directory)
because virhostdevtest still tests the old KVM assignment and
thus has no notion of IOMMU groups. This will change in near
future though. And in order to discover devices belonging to the
same IOMMU group we need to do what kernel does - create symlinks
to devices.

Signed-off-by: Michal Privoznik <mprivozn@redhat.com>
Tested-by: Daniel Henrique Barboza <danielhb413@gmail.com>
Reviewed-by: Daniel Henrique Barboza <danielhb413@gmail.com>