schema: add password configuration for ssh disk

schema: add password configuration for ssh disk

Right now, ssh network disks are not usable. There is some basic support
in libvirt that is meant to support disk chains that have backing disks
located at ssh urls, but there is no real way for a user to configure a
ssh-based disk.  This commit allows users to configure an ssh disk with
password authentication. Implementation will follow.

<disk type='network'>
  <source  protocol='ssh' ...>
    <auth username='myusername'>
      <secret type='iscsi' usage='secretname'/>
    </auth>
</disk>

Signed-off-by: Jonathon Jongsma <jjongsma@redhat.com>
Reviewed-by: Peter Krempa <pkrempa@redhat.com>