diskunmap vdsm hook:
==================
This hook goes through all of the VM's disks and manipulate its XML
file acccording to the input. This can be used to enable UNMAP feature
to discard unused blocks for better use of thin provisioned storage
devices.

Syntax:
    diskunmap=(off|on)

Where:
    'on' is unmap enabled and off (default) is unmap disabled.

Example:
    diskunmap=on

Installation:
    - Use the engine-config to append the proper custom property:
        $ sudo engine-config -s UserDefinedVMProperties='diskunmap=^(off|on)$'
    - Verify that the macbind custom property was properly added:
        $ sudo engine-config -g UserDefinedVMProperties

Usage:
    In the VM configuration window, open the custom properites tab, select
    diskmap and select 'on', activating disk UNMAP for all disks and LUNs.
    Only devices using IDE or VirtIO-SCSI interface will be affected. UNMAP is
    not available for VirtIO interface.

Expected Result:
    For every DISK or LUN device, this configuration will include
    "discard=unmap" to disk driver xml:

    <disk device="disk" snapshot="no" type="block">
        ...
        <driver cache="none" discard="unmap" ...  />
    </disk>

    This option will be translated to qemu as bellow:

    ... -drive file=<file>,discard=unmap,...
