libxl: suspend: Async xenstore pvcontrol wait

libxl: suspend: Async xenstore pvcontrol wait

When negotiating guest suspend via the xenstore pvcontrol protocol
(ie when the guest does NOT support the evtchn fast suspend protocol):

Replace the use of loops and usleep with a call to libxl__xswait.

Also, replace the xenstore transaction loop with one using
libxl__xs_transaction_start et al.

There is not intended to be any semantic change, other than to make
the algorithm properly asynchronous.

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: Add a comment to clarify last minute ack.
v3X: Do NOT rename "pvcontrol" xswait state struct to "guest_wait"
     (because we're NOT going to use it for the event channel based wait
     too).