DiskDevice API v2.4 remarks

Howdy,

some further remarks from me.

* BPartition* BDiskDeviceJob::Partition(): There is no BPartition 
object, since we only have te partition ID. We would need to allocate a 
complete BDiskDevice hierarchy to be able to return a BPartition. I'd 
prefer to just return the partition ID (perhaps rename the method to 
PartitionID()?). If the caller wants an object, there's 
BDiskDeviceRoster::GetPartitionWithID().

* A BDiskDeviceJob type is missing: B_DISK_DEVICE_JOB_SET_PARAMETERS.

* Would watching disk systems make sense? We'll probably have to do 
that in the kernel anyway, so it would take little to implement it.

* BDiskDevice::CommitModifications(): I know, I know, I proposed it, 
but I don't think the template message is that good an idea. It would 
mean passing a flattened version of it to the kernel and then further 
to the registrar (if we indeed implement delivery of kernel 
notification messages there). For rather little benefit, I believe.

CU, Ingo