Changes common to all network disks:
-Make source name optional in the domain schema, since NBD doesn't use it
-Add a hostName type to the domain schema, and use it instead of genericName, which doesn't include .
-Don't leak host names or ports
-Set the source protocol in qemuParseCommandline
Signed-off-by: NJosh Durgin <joshd@hq.newdream.net>