conf: add passt XML additions to schema

conf: add passt XML additions to schema

Initial support for network devices using passt (https://passt.top)
for the backend connection will require:

* new attributes of the <backend> subelement:
  * "type" that can have the value "passt" (to differentiate from
    slirp, because both slirp and passt will use <interface
    type='user'>)
  * "logFile" (a path to a file that passt should use for its logging)
  * "upstream" (a netdev name, e.g. "eth0").

* a new subelement <portForward> (described in more detail later)

Signed-off-by: Laine Stump <laine@redhat.com>
Reviewed-by: Ján Tomko <jtomko@redhat.com>