tools/libxl: Stash all restore parameters in domain_create_state

tools/libxl: Stash all restore parameters in domain_create_state

Shortly more parameters will appear, and this saves unboxing each one.
libxl_domain_restore_params is mandatory for restore streams, and
ignored for plain creation.  The old 'checkpointed_stream' was
incorrectly identified as a private parameter when it was infact
public.

No functional change.

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Acked-by: Ian Campbell <Ian.Campbell@citrix.com>
Reviewed-by: Yang Hongyang <yanghy@cn.fujitsu.com>
CC: Ian Jackson <Ian.Jackson@eu.citrix.com>
CC: Wei Liu <wei.liu2@citrix.com>
---
Since v1:
 * Gate validity on restore_fd being valid.