libxl: suspend: Async evtchn wait

libxl: suspend: Async evtchn wait

When negotiating guest suspend via the evtchn ("fast") protocol,
abolish synchronous wait for domain suspend.

If the guest supports the event channel suspend protocol, we used to
sit in a loop in xc_await_suspend waiting (perhaps indefinitely) for
it to suspend.

Instead, use the new libxl event channel event facility.  When we see
that the event is signaled, we look at the domain to see if it has
suspended.  (In this patch we do not yet set a timeout; that will come
next.)

So the suspend operation no longer blocks with the libxl ctx lock
held, and instead returns to the event loop.  Additionally, domains
which signal the event channel themselves, or undergo other state
changes, will be handled more correctly.

We end up making a few more hypercalls.

Also, if we encounter errors setting up the suspend event channel
(which should not happen), abort the operation rather than falling
back to the xenstore protocol.

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>
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v3: Improve commit message.