qga: Introduce NVMe disk bus type

Assigning a NVMe disk by VFIO or emulating a NVMe controller by QEMU,
a NVMe disk get exposed in guest side. Support NVMe disk bus type and
implement posix version.

Test PCI passthrough case:
~#virsh qemu-agent-command buster '{"execute":"guest-get-disks"}' | jq
  ...
    {
      "name": "/dev/nvme0n1",
      "dependencies": [],
      "partition": false,
      "address": {
        "serial": "SAMSUNG MZQL23T8HCLS-00A07_S64HNE0N500076",
        "bus-type": "nvme",
        "bus": 0,
        "unit": 0,
        "pci-controller": {
          "bus": 0,
          "slot": 22,
          "domain": 0,
          "function": 0
        },
        "dev": "/dev/nvme0n1",
        "target": 0
      }
  ...

Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Signed-off-by: zhenwei pi <pizhenwei@bytedance.com>
Message-Id: <20220420022610.418052-2-pizhenwei@bytedance.com>