libxl/save: Refactor libxl__domain_suspend_state

libxl/save: Refactor libxl__domain_suspend_state

Currently struct libxl__domain_suspend_state contains 2 type of states,
one is save state, another is suspend state. This patch separates those
two out.
The motivation of this is that COLO will need to do suspend/resume
continuously, we need a more common suspend state.

After this change, dss stands for libxl__domain_save_state,
dsps stands for libxl__domain_suspend_state.

Also introduce libxl__domain_suspend_init to initialise the
libxl__domain_suspend_state.

Signed-off-by: Yang Hongyang <hongyang.yang@easystack.cn>
Signed-off-by: Wen Congyang <wency@cn.fujitsu.com>
CC: Ian Campbell <Ian.Campbell@citrix.com>
CC: Ian Jackson <Ian.Jackson@eu.citrix.com>
CC: Wei Liu <wei.liu2@citrix.com>
CC: Andrew Cooper <andrew.cooper3@citrix.com>
Acked-by:Ian Campbell <ian.campbell@citrix.com>
Reviewed-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
Acked-by: Wei Liu <wei.liu2@citrix.com>