Add reporting of disk serial number on Linux guests. The feature depends
on libudev.

Example:

    {
      "name": "dm-2",
      "mountpoint": "/",
      ...
      "disk": [
        {
          "serial": "SAMSUNG_MZ7LN512HCHP-000L1_S1ZKNXAG822493",
          ...
        }
      ],
    }

Signed-off-by: Tomáš Golembiovský <tgolembi@redhat.com>
Signed-off-by: Michael Roth <mdroth@linux.vnet.ibm.com>