qemuBuildVhostuserCommandLine: Unify -netdev creation
    
    Currently, what we do for vhost-user network is generate the
    following part of command line:
    
    -netdev type=vhost-user,id=hostnet0,chardev=charnet0
    
    There's no need for 'type=' it is the default. Drop it.
    
    Signed-off-by: Michal Privoznik <mprivozn@redhat.com>