Tweak EOF handling of streams

Tweak EOF handling of streams

Typically when you get EOF on a stream, poll will return
POLLIN|POLLHUP at the same time. Thus when we deal with
stream reads, if we see EOF during the read, we can then
clear the VIR_STREAM_EVENT_HANGUP & VIR_STREAM_EVENT_ERROR
event bits.

Signed-off-by: Daniel P. Berrange <berrange@redhat.com>