libxl: suspend: Reorg domain_suspend_callback_common

libxl: suspend: Reorg domain_suspend_callback_common

Make domain_suspend_callback_common more callback-oriented:

* Turn the functionality behind the goto labels "err" and
  "guest_suspended" into functions which can be called just before
  "return".

* Deindent the "issuing %s suspend request via XenBus control node"
  branch; it is going to be split up into various functions as the
  xenstore work becomes callback-based.

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>