engine: Moving Duplicate Code to Base Class

This patch moves the duplicate code from AddVmInterfaceCommand
and UpdateVmInterfaceCommand into their base class.

Change-Id: I3b07249bc2e554e2a420792fd100781dff748958
Bug-Url: https://bugzilla.redhat.com/905844
Signed-off-by: Muli Salem <msalem@redhat.com>