vfio/pci: Remove dev_fmt definition

It currently results in messages like:

 "vfio-pci 0000:03:00.0: vfio_pci: ..."

Which is quite a bit redundant.

Reviewed-by: Cornelia Huck <cohuck@redhat.com>
Reviewed-by: Kevin Tian <kevin.tian@intel.com>
Signed-off-by: Alex Williamson <alex.williamson@redhat.com>