io: cope with websock 'Connection' header having multiple values

The noVNC server sends a header "Connection: keep-alive, Upgrade" which
fails our simple equality test. Split the header on ',', trim whitespace
and then check for 'upgrade' token.

Reviewed-by: Eric Blake <eblake@redhat.com>
Signed-off-by: Daniel P. Berrange <berrange@redhat.com>
