Fix video on PCI heuristic

The video on PCI heuristic was broken. It was supposed to infer a
video device when the last element of the path was a PCI DEVICE PATH
node. However, the last node in the device path is an END node, so
this heuristic never fired.

This leads, among other things, to bhyve only producing output in the
serial connection once we leave the boot loader. This restores the
dual headed boot on bhyve + UEFI (as we did in 11.2), but will favor
serial in the absence of other config which may be a change from 11.2.

MFC After: 3 days
Differential Revision: https://reviews.freebsd.org/D26572