nvme errorneously reports success if guests tries to disable volatile write cache

[luqmana]

We always return success if the guest tries to update the Volatile Write Cache feature:

propolis/lib/propolis/src/hw/nvme/admin.rs

Lines 276 to 283 in 803cb36

	cmds::FeatureIdent::VolatileWriteCache => {
	// NVMe 1.0e Figure 66 Identify - Identify Controller Data
	// Structure "If a volatile write cache [VWC] is present, then
	// the host may ... control whether it is enabled with Set
	// Features specifying the Volatile Write Cache feature
	// identifier."
	cmds::Completion::success()
	}

This can potentially cause data loss as the guest will assume that write will be committed on completion.