parse nbd_protocol_version if available

parse nbd_protocol_version if available

For secure live migration with local disks via NBD over a unix socket,
we have to somehow communicate from the source node to the target node
if it supports it. This is because there can only be one NBD server with
exactly one socket bound.

The source node passes that information via STDIN. Support for
'spice_ticket: (...)' is added in addition to 'nbd_protocol_version:
<version>'. As old source nodes send the spice ticket without a prefix,
we still have to have a fallback for this case. New information should
always be passed via a prefix that is matched, otherwise it will be
recognized as spice ticket.

Signed-off-by: Mira Limbeck <m.limbeck@proxmox.com>