qga: add implementation of guest-get-disks for Windows

The command lists all the physical disk drives. Unlike for Linux
partitions and virtual volumes are not listed.

Example output:

{
  "return": [
    {
      "name": "\\\\.\\PhysicalDrive0",
      "partition": false,
      "address": {
        "serial": "QM00001",
        "bus-type": "sata",
        ...
      },
      "dependents": []
    }
  ]
}

Signed-off-by: Tomáš Golembiovský <tgolembi@redhat.com>
Signed-off-by: Michael Roth <michael.roth@amd.com>