- allow vnddetach() to return EBUSY if any vnd's are currently initialised.
lkm:
- add new 'dev' directory, initially with just a 'vnd' LKM. for now, the
vnd lkm driver requests 4 devices....
XXX: vnd should be converted to a psuedo-device that creates & deletes
instances of itself (vnd0, vnd1, etc) when vnconfig -c/-u are called,
then the vnd lkm driver can not be limited to '4' by default.
