packaging: setup: copy virtio files if they exist

Current implementation didn't load the image_upload
plugin correctly. Additionally, the plugin itself
checked its own relevance before NFS export was created.

New implementation loads the plugin correctly and moves
NFS check to the validation stage, where NFS creation/export
configuration step already have passed.

Change-Id: Ie35c5a3ad38b2f7bdae88c1f8d0790d1a8c988c2
Signed-off-by: Alex Lourie <alourie@redhat.com>