core,webadmin: Enable virtual drive size extension

This patch provides the user the ability to extend a virtual
disk size online (when VM is running or paused) and offline
(when VM is down or suspended).

This functionality is exposed through the REST API and UI:
in UI -
    go to "Virtual Machines" tab and choose the VM
    go to the "Disks" sub tab and choose the disk
    click on "Edit" and insert value into "Extend size by(GB)" input
    field

REST API -
    Size is the property of the disk and can be updated like any other
    property by sending "PUT" request to:

    /api/vms/{VM_ID}}/disks/{DISK_ID}

    <disk>
        <size>{NEW_SIZE}</size>
    </disk>

Change-Id: Ie702348a68a26ac02a01f66aaa1ea42c2c675ebb
Signed-off-by: Sergey Gotliv <sgotliv@redhat.com>