tools/libxl: move domain suspend code into libxl_dom_suspend.c

tools/libxl: move domain suspend code into libxl_dom_suspend.c

Move domain suspend code into a separate file libxl_dom_suspend.c.
Add an API libxl__domain_suspend() which wraps the static
function domain_suspend_callback_common() for internal use.
Export the existing API libxl__domain_suspend_callback() used by
libxc to suspend the guest during migration.

Note that the newly added file libxl_dom_suspend.c is used for
suspend/resume code.

Signed-off-by: Yang Hongyang <yanghy@cn.fujitsu.com>
CC: Ian Jackson <Ian.Jackson@eu.citrix.com>
CC: Wei Liu <wei.liu2@citrix.com>
Acked-by: Ian Campbell <Ian.Campbell@citrix.com>