nvme(4): Add MSI and single MSI-X support.

If we can't allocate more MSI-X vectors, accept using single shared.
If we can't allocate any MSI-X, try to allocate 2 MSI vectors, but
accept single shared. If still no luck, fall back to shared INTx.

This provides maximal flexibility in some limited scenarios. For
example, vmd(4) does not support INTx and can handle only limited
number of MSI/MSI-X vectors without sharing.

MFC after: 1 week