openvmm: emulated nvme configuration

[VivianNK]

It would be helpful for testing to add configuration options to emulated nvme devices. Some of these configurations that QEMU supports are here (https://www.qemu.org/docs/master/system/devices/nvme.html)

For my testing purposes I'm interested in:

1. Adding max_ioqpairs=UINT32 (default: 64)
   This would allow setting the maximum number of controller allowed I/O queue pairs

2. Support additional NVMe controllers. Currently, adding NVMe drives adds additional namespaces

3. Supporting qcow2 files

[jstarks]

All good ideas.

The first 2 are supported by the platform but are not exposed via the CLI. If you want to propose the CLI syntax for this, it should be easy to implement.

qcow2 support is more work. But if you can find a crate that supports qcow2 that we can reuse, then it shouldn't be too hard to integrate (at some perf penalty vs. what's optimal, since 3p crates probably don't support async).