libxl: suspend: common suspend callbacks take rc

libxl: suspend: common suspend callbacks take rc

Change the following functions to take a libxl error code rather than
a boolean "ok" value, and translate that value to the boolean expected
by libxc at the last moment:
  domain_suspend_callback_common_done        } dss->callback_common_done
  remus_domain_suspend_callback_common_done  }
  domain_suspend_common_done

Also, abolish domain_suspend_common_failed as
domain_suspend_common_done can easily do its job and the call sites
now have to supply the right rc value anyway.

In domain_suspend_common_guest_suspended, change "ret" to "rc"
as it contains a libxl error code.

There is no functional change in this patch: the proper rc value now
propagates further, but is still eventually smashed to a boolean.

Signed-off-by: Ian Jackson <Ian.Jackson@eu.citrix.com>
Acked-by: Ian Campbell <ian.campbell@citrix.com>
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v2: Fix a leftover comment referring to domain_suspend_common_failed