Libvirt-Enable suppport for discard option for disk device

Support setting discard option for instance's disk.
If discard option is enabled, unused/freed blocks can be return back to the
storage device.

blueprint libvirt-disk-discard-option

Change-Id: Idcf7b35c93422d0d13c3ffb33aca3c5eca4f3c94
Merge "Libvirt-Enable suppport for discard option for disk device"