libvirt's network config documents that a bridge's STP "forward delay"
(called "delay" in the XML) should be specified in seconds, but
virNetDevBridgeSetSTPDelay() assumes that it is given a delay in
milliseconds (although the comment at the top of the function
incorrectly says "seconds".

This fixes the comment, and converts the delay to milliseconds before
calling virNetDevBridgeSetSTPDelay().