Thank you very much for your help !

I forgot question no.6: is it impossible to use save/restore or I'm doing 
something wrong ? Here is what I'm doing:

xen0-1# xm list
Name              Id  Mem(MB)  CPU  State  Time(s)  Console
Domain-0           0       31    0  r----    261.5
nbsd               3       31    0  -b---   3976.9    9603
nbsd2              2       31    0  -b---     37.2    9602
xen0-1# xm save nbsd2 test
Error: errors: transfer daemon (xfrd) error: 1
xen0-1# cat /var/log/xfrd.log
338 [INF] XFRD> Accepted connection from 127.0.0.1:65459 on 3
338 [INF] XFRD> Forked child pid=622
622 [INF] XFRD> Xfr service for 127.0.0.1:65459
[DEBUG] Conn_init> flags=1
[DEBUG] Conn_init> write stream...
[DEBUG] stream_init>mode=w flags=1 compress=0
[DEBUG] stream_init> unbuffer...
[DEBUG] stream_init< err=0
[DEBUG] Conn_init> read stream...
[DEBUG] stream_init>mode=r flags=1 compress=0
[DEBUG] stream_init> unbuffer...
[DEBUG] stream_init< err=0
[DEBUG] Conn_sxpr>
(xfr.hello 1 0)[DEBUG] Conn_sxpr< err=0
[DEBUG] Conn_sxpr>
(xfr.save 2 '(domain (id 2) (name nbsd2) (memory 31) (maxmem 32) 
(state -b---) (cpu 0) (cpu_time 37.323428053) (up_time 18003.3614259) 
(start_time 1136461423.33) (console (status listening) (id 16) (domain 2) 
(local_port 16) (remote_port 1) (console_port 9602)) (devices (vif (idx 0) 
(vif 0) (mac aa:00:00:00:02:e0) (vifname vif2.0) (bridge bridge0) (evtchn 17 
4) (index 0)) (vbd (idx 0) (vdev 11) (device 7) (mode w) (dev wd1d) (uname 
phy:/dev/wd0h) (node /dev/wd0h) (index 0)) (vbd (idx 1) (vdev 3) (device 6) 
(mode w) (dev wd0d) (uname phy:/dev/wd0g) (node /dev/wd0g) (index 1))) 
(config (vm (name nbsd2) (memory 32) (image (linux (kernel 
/xen-kernels/netbsd-XENU) (root /dev/wd0d))) (device (vbd (uname 
phy:/dev/wd0h) (dev wd1d) (mode w))) (device (vbd (uname phy:/dev/wd0g) (dev 
wd0d) (mode w))) (device (vif (mac aa:00:00:0:02:e0) (bridge bridge0))))))' 
/var/log/test)[DEBUG] Conn_sxpr< err=0
[1136479426.907399] xc_linux_save start 2

xc_linux_save start 2
Couldn't map pfn_to_mfn_frame_list
622 [INF] XFRD> Xfr service err=0
Couldn't map pfn_to_mfn_frame_list

-- 
Thanks,
Mihai