error "could not open disk image" and snapshot=on (if off it works)

Ciao,
I have this strange problem:
ubuntu 8.10, kvm 72 2.6.27-7-server x86_64 GNU/Linux

vdeq kvm -name proxy_UBUNTU_8.04 \
-net nic,macaddr=00:16:3e:00:a0:00 -net
tap,ifname=tap1,script=no,downscript=no \
-net nic,macaddr=00:16:3e:00:a1:01,vlan=1 -net vde,vlan=1,sock=/var/run/vde2/tun1.ctl \
-drive file=./ubuntu-server-8.04_proxy.root,if=scsi,index=0,snapshot=off,cache=on,boot=on \
-drive file=./ubuntu-server-8.04_proxy.home,if=scsi,index=1,snapshot=off,cache=on \
-drive file=./linux.swap,if=scsi,index=2,cache=on,snapshot=on \
-smp 1 -M pc -cpu pentium3 -m 512 -k en-us -localtime

qemu: could not open disk image ./linux.swap

but

$ ls -l linux.swap
-rw-rw-r-- 1 paolop virtual 1073741824 2008-12-01 13:21 linux.swap

exist

If I set "snapshot=off" on linux.swap, kvm boot without problems. (the same happened with other file-device)
So it's seems a "wrong error message", it's not a filesystem problem but an option problem
(snapshot=on doesn't work, snapshot=off works)

Any suggestion?
thank you.

Paolo Pedaletti