vm payload: creation of full pathnames in payload

Change required for cloud-init integration: support the creation of
payloads with files residing in subdirectories of the payload image.

Change-Id: Ifd76b5d9a3b79126dfcb764ac368a3631d743d1c
Bug-Url: https://bugzilla.redhat.com/619761
Signed-off-by: Greg Padgett <gpadgett@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/16072
Reviewed-by: Dan Kenigsberg <danken@redhat.com>