libxl: add libxl_domain_suspend_only to simply suspend a domain, without saving it

libxl: add libxl_domain_suspend_only to simply suspend a domain, without saving it

Similar functionality to libxl_domain_suspend(), but do not save domains
state to any file. Only suspend the domain and keep it in suspended
shutdown state (do not destroy it). Such domain can be later woken up
with libxl_domain_resume. The main reason for this functionality is to
suspend the host while some domains are running, potentially holding PCI
devices. This will give a chance to a driver in such a domain to
properly suspend the device.

Signed-off-by: Marek Marczykowski-Górecki <marmarek@invisiblethingslab.com>
Signed-off-by: Marcus of Wetware Labs <marcus@wetwa.re>
Acked-by: Wei Liu <wei.liu2@citrix.com>