virsh migrate w/o shared storage

My mission: migration from iron3 -> iron4 without using shared storage.

# first see my sparse file

[root@iron3 images]# ls -lAhs
[...snip]
7.6G -rwxr-xr-x. 1 root root 500G Apr 29 18:51 ncr.raw

# apparently you need to "prepare" the destination. if not, the migrate fails with:
error: unable to set user and group to '107:107' on '/var/lib/libvirt/images/ncr.raw': No such file or directory
# should I be doing this, or is there a better way ? Please note, this new image *is* sparse (and empty).

[root@iron4 images] time qemu-img create -f raw ncr.raw 500G
Formatting 'ncr.raw', fmt=raw size=536870912000

# okay, go time!

[root@iron3 ~]# time virsh migrate --verbose --live --copy-storage-all ncr qemu+ssh://iron4/system
Migration: [  0 %]
...
[...finishes successfully after 78 minutes!]

# note: this took so long because it didn't copy it in a "sparse" way (such as rsync -S can)
[root@iron4 images]# ls -lAhs
total 501G
501G -rw-r--r-- 1 qemu qemu 500G Apr 29 21:04 ncr.raw

[root@iron4 images]# df -h
Filesystem            Size  Used Avail Use% Mounted on
/dev/mapper/vg_00-lv_root
                      621G 1009M  588G   1% /
tmpfs                  24G     0   24G   0% /dev/shm
/dev/sda1             504M   42M  437M   9% /boot
/dev/mapper/vg_00-lv_home
                      504M   17M  462M   4% /home
/dev/mapper/vg_00-lv_var
                     1008G  501G  457G  53% /var

# I can "fix" the image with qemu-img, but ideally it should stay sparse to begin with! What have I done wrong?
[root@iron4 images]# mv ncr.raw ncr.raw.original
[root@iron4 images]# time qemu-img convert ncr.raw.original ncr.raw

Many thanks in advance,
James