xenopsd: avoid log message about vmdesc

QEMU complains during a migration:
```
qemu-dm-120: Expected vmdescription section, but got 0
```

This has been fixed upstream in 2020, but only for libxl:
https://patchew.org/Xen/[email protected]/

(And QEMU 5.2+ would do it automatically for machine=xenfv but we're using machine=pc,
and an older QEMU.
"machine=pc" is better and offers more flexibility in setting up a vm than "xenfv" offers.)

Implement the equivalent fix for xenopsd: we use qemu 4.x, and this CLI flag
first appeared in 2.3, so should be fine.

Signed-off-by: Edwin Török <[email protected]>

ocaml/xenopsd/scripts/qemu-wrapper

@@ -154,6 +154,7 @@ def main(argv):
 
     qemu_args.extend(['-machine',
                       '%s,accel=xen,max-ram-below-4g=%lu,'
+                      'suppress-vmdesc=on,'
                       'allow-unassigned=true,trad_compat=%s%s'
                       % (machine, mmio_start, trad_compat, igdpt)])