engine: Clearing address of vm disk device if it's interface was changed.

Updating disk interface its like detach and attach disk again, therefore
address of the vm device representing this disk must be cleared.

Change-Id: I62605c490da909447f77513e7691d76ddd24ff26
Bug-Url: https://bugzilla.redhat.com/994247
Signed-off-by: Sergey Gotliv <sgotliv@redhat.com>