doc: Add limitation of Virtual Network connection recovery in DomU

The virtual network connection is not recoverable after the domU
guest reset. This limitation is added to the changelog document.

Issue-Id: SCM-5631
Signed-off-by: Yanqin Wei <Yanqin.Wei@arm.com>
Change-Id: I70059881b302d0c132d487e12cbe29c199b7719f