stand/ofw: ofw_disk isn't really a disk

The rest of the code in the tree assumes that a DEVT_DISK uses a
disk_devdesc to represent the device. However ofw_disk diesn't, so we
can't use disk_fmtdev, nor disk_parsedev. ofw needs to have a
dv_match-like routine to use devpasrse, though, since we have two
drivers (net and block) that claim the same sort of devices (eg
/path/to/ofw-dev) based on the device-type property. In the interim, we
can't use devmatch and ofw_disk's and the default net driver's parsing
is offloaded ofw_parsedev.

Sponsored by: Netflix
Differential Revision: https://reviews.freebsd.org/D37347