libxl: suspend: Async domain_suspend_callback_common

libxl: suspend: Async domain_suspend_callback_common

Make domain_suspend_callback_common do its work and then call
dss->callback_common_done, rather than simply returning its answer.

This is preparatory to abolishing the usleeps in this function and
replacing them with use of the event machinery.

No functional change.

Signed-off-by: Ian Jackson <Ian.Jackson@eu.citrix.com>
CC: Stefano Stabellini <stefano.stabellini@eu.citrix.com>
Acked-by: Ian Campbell <ian.campbell@citrix.com>
---
v3: Remove some trailing whitespace