https://bugzilla.redhat.com/show_bug.cgi?id=1322717

During offline migration, no storage is copied. Nor disks, nor
NVRAM file, nor anything. We use qemu for that and because domain
is not running there's nobody to copy that for us.
We should document this to avoid confusing users.
Signed-off-by: NMichal Privoznik <mprivozn@redhat.com>