ueventd: Fix creation of VFIO dev nodes

VFIO nodes, both the container (`vfio`) node and group (numbered)
nodes, should be located in `/dev/vfio`. This change prevents
ueventd from flattening that structure.

Test: Bind a device to VFIO driver to create a VFIO group
Change-Id: I635e9febe6bb52718df263e735479f361eacad4c