qga: add command guest-get-devices for reporting VirtIO devices

qga: add command guest-get-devices for reporting VirtIO devices

Add command for reporting devices on Windows guest. The intent is not so
much to report the devices but more importantly the driver (and its
version) that is assigned to the device. This gives caller the
information whether VirtIO drivers are installed and/or whether
inadequate driver is used on a device (e.g. QXL device with base VGA
driver).

Example:
[
    {
      "driver-date": "2019-08-12",
      "driver-name": "Red Hat VirtIO SCSI controller",
      "driver-version": "100.80.104.17300",
      "address": {
        "type": "pci",
        "data": {
          "device-id": 4162,
          "vendor-id": 6900
        }
      }
    },
    ...
]

Signed-off-by: Tomáš Golembiovský <tgolembi@redhat.com>
Reviewed-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
*remove redundant glib autoptr declaration for GuestDeviceInfo
Signed-off-by: Michael Roth <mdroth@linux.vnet.ibm.com>