Adding make_dev() and destroy_dev() calls in (hopefully) the right
places.
This is done by calling make_dev() in each object constructor and
caching the dev_t's returned from make_dev() in each struct
'subdisk'(sd), 'plex' and 'volume' such that the 'object'_free()
functioncs can call destroy dev.
This change makes a subset of the old /dev/vinum appear under devfs.
Enough nodes appear such that I'm able to mount my striped volume.
There may be more work needed to get vinum configuration working
properly.
d9ca06d2a3