net: handle QIOTask completion to report useful error message

net: handle QIOTask completion to report useful error message

The network stream backend uses the async QIO socket APIs for listening
and connecting sockets. It does not check the task object completion
status, however, instead just looking at whether the socket FD is -1
or not.

By checking the task completion, we can set a useful error message for
users instead of the non-actionable "connection error" string.

eg so users will see:

(qemu) info network
net: index=0,type=stream,error: Failed to connect to '/foo.unix': No such file or directory

Signed-off-by: "Daniel P. Berrangé" <berrange@redhat.com>
Message-ID: <20240104162942.211458-6-berrange@redhat.com>
Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
Signed-off-by: Thomas Huth <thuth@redhat.com>