vm payload: clarify intent of supporting multiple files

With cloud-init integration now using payloads with multiple files,
clarify that this is supported in the payload code.

Change-Id: I3b2a9828eb1fee5bc5c25d5faa90b387473e820e
Bug-Url: https://bugzilla.redhat.com/619761
Signed-off-by: Greg Padgett <gpadgett@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/16073
Reviewed-by: Dan Kenigsberg <danken@redhat.com>
Tested-by: Dan Kenigsberg <danken@redhat.com>