conf: parse/format passt-related XML additions

conf: parse/format passt-related XML additions

This implements XML config to represent a subset of the features
supported by 'passt' (https://passt.top), which is an alternative
backend for emulated network devices that requires no elevated
privileges (similar to slirp, but "better").

Along with setting the backend to use passt (via <backend
type='passt'/> when the interface type='user'), we also support
passt's --log-file and --interface options (via the <backend>
subelement logFile and upstream attributes) and its --tcp-ports and
--udp-ports options (which selectively forward incoming connections to
the host on to the guest) via the new <portForward> subelement of
<interface>. Here is an example of the config for a network interface
that uses passt to connect:

    <interface type='user'>
      <mac address='52:54:00:a8:33:fc'/>
      <ip address='192.168.221.122' family='ipv4'/>
      <model type='virtio'/>
      <backend type='passt' logFile='/tmp/xyzzy.log' upstream='eth0'/>
      <portForward address='10.0.0.1' proto='tcp' dev='eth0'>
        <range start='2022' to='22'/>
        <range start='5000' end='5099' to='1000'/>
        <range start='5010' end='5029' exclude='yes'/>
      </portForward>
      <portForward proto='udp'>
        <range start='10101'/>
      </portForward>
    </interface>

In this case:

* the guest will be offered address 192.168.221.122 for its interface
  via DHCP

* the passt process will write all log messages to /tmp/xyzzy.log

* routes to the outside for the guest will be derived from the
  addresses and routes associated with the host interface "eth0".

* incoming tcp port 2022 to the host will be forwarded to port 22
  on the guest.

* incoming tcp ports 5000-5099 (with the exception of ports 5010-5029)
  to the host will be forwarded to port 1000-1099 on the guest.

* incoming udp packets on port 10101 will be forwarded (unchanged) to
  the guest.

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