A late change to the SR-IOV infrastructure broke passthrough of
VFs. device_set_devclass() was being used to try to force the
ppt driver to attach to the device, but this didn't work because
the DF_FIXEDCLASS flag wasn't being set on the device, so the
ppt driver probe routine would not match when it returned
BUS_NOWILDCARD. Fix this by adding a new device function that
both sets the devclass and sets the DF_FIXEDCLASS flag, and use
that to force the ppt driver to attach to VFs.
Differential Revision: https://reviews.freebsd.org/D2041
Reviewed by: jhb
MFC after: 3 weeks