Add back an important synchronisation point that had fallen off
Add back an important synchronisation point that had fallen off

This was breaking suspend/resume/migrate on windows guests with
PV drivers.

Signed-off-by: Jon Ludlam <e7e3380887a8f95cc9dc4f0d51dedc7e849a287a@eu.citrix.com>

thomassa/xenopsd,djs55/xenopsd-previous,djs55/xenopsd,koushikcgit/xenopsd,djs55/xenopsd-previous,djs55/xenopsd-previous,euanh/xenopsd,johnelse/xenopsd,koushikcgit/xenopsd,simonjbeaumont/xenopsd,thomassa/xenopsd,cheng--zhang/xenopsd,johnelse/xenopsd,euanh/xenopsd,cheng--zhang/xenopsd,johnelse/xenopsd,simonjbeaumont/xenopsd,simonjbeaumont/xenopsd,koushikcgit/xenopsd,cheng--zhang/xenopsd,djs55/xenopsd,euanh/xenopsd,djs55/xenopsd,thomassa/xenopsd