allow stream download from path and over pvedaemon-proxy

allow stream download from path and over pvedaemon-proxy

Allow specifying a filepath for stream=1 instead of either a path or fh
with stream=1.

With this in place, we can also just return the path to the proxy in
case we want to stream a response back, and let it read from the file
itself. This way, the pvedaemon is cut out of the transfer pipe.

Signed-off-by: Stefan Reiter <s.reiter@proxmox.com>