This is especially the case for domain-shutdown event for DispVM - when
handled, DispVM can be already removed. Give the handled VM object even
if actual VM is already removed. For this, avoid VM existence check by
using domains.get_blind() method.
And actually implement domains.get_blind() method - it was present only
in generic collection, but not VMCollection.
Empty parameter value is encoded as b'parameter\0\0', so we can't simply
read the data until b'\0\0', because it isn't necessary event end.
Instead, read event parts separately, according to specification.
