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

The command lists all disks (real and virtual) as well as disk
partitions. For each disk the list of dependent disks is also listed and
/dev path is used as a handle so it can be matched with "name" field of
other returned disk entries. For disk partitions the "dependents" list
is populated with the the parent device for easier tracking of
hierarchy.

Example output:
{
  "return": [
    ...
    {
      "name": "/dev/dm-0",
      "partition": false,
      "dependents": [
        "/dev/sda2"
      ],
      "alias": "luks-7062202e-5b9b-433e-81e8-6628c40da9f7"
    },
    {
      "name": "/dev/sda2",
      "partition": true,
      "dependents": [
        "/dev/sda"
      ]
    },
    {
      "name": "/dev/sda",
      "partition": false,
      "address": {
        "serial": "SAMSUNG_MZ7LN512HCHP-000L1_S1ZKNXAG822493",
        "bus-type": "sata",
        ...
        "dev": "/dev/sda",
        "target": 0
      },
      "dependents": []
    },
    ...
  ]
}

Signed-off-by: Tomáš Golembiovský <tgolembi@redhat.com>
Reviewed-by: Marc-André Lureau <marcandre.lureau@redhat.com>
*add missing stub for !defined(CONFIG_FSFREEZE)
*remove unused deps_dir variable
Signed-off-by: Michael Roth <michael.roth@amd.com>