VM shutdowns: do not block the calls to the domains prematurely

[psafont]

I'm opening the PR to comment on this behaviour, since this should make possible to shutdown FreeBSD kernels.

The crux of the matter is that Just because a domain is not advertising it can shutdown cooperatively doesn't mean it can't at all. Doing it in any case would make it work with FreeBSD kernels, but I don't know whether this can break some expectations in xencenter about other guests.

[andyhhp]

The feature-$foo flags were added a decade after Xen support had found its way into various open source kernels.

While the presence of explicit =1 or =0 values in the feature flags does indicate a guest aware of the new scheme, the absence of features does not indicate that the guest is Xen-unaware.

[lindig]

I believe the commit should explain the semantics of the strict flag; what happens in both cases and what we are using now.

[psafont]

I can add an explanation to the strict mode in the call. I believe it's misnamed, the parameter enforces that the guest is actively advertising support for the feature using an appropriate xenstore key. This is not required by the spec, and it's not clear whether not enforcing could break windows guests, hence the PR being in draft.