tools/libxl: only set viridian flags on new domains

tools/libxl: only set viridian flags on new domains

Domains migrating or restoring should have viridian HVM param key in
the migration stream already and setting that twice results in Xen
returing -EEXIST on the second attempt later (during migration stream parsing)
in case the values don't match. That causes migration/restore operation
to fail at destination side.

That issue is now resurfaced by the latest commits (983524671 and 7e5cffcd1e)
extending default viridian feature set making the values from the previous
migration streams and those set at domain construction different.

Suggested-by: Andrew Cooper <andrew.cooper3@citrix.com>
Signed-off-by: Igor Druzhinin <igor.druzhinin@citrix.com>
Reviewed-by: Andrew Cooper <andrew.cooper3@citrix.com>
Release-Acked-by: Ian Jackson <iwj@xenproject.org>