qga/commands-posix: Fix listing ifaces for Solaris

The code for guest-network-get-interfaces needs a couple of small
adjustments for Solaris:

- The results from SIOCGIFHWADDR are documented as being in ifr_addr,
  not ifr_hwaddr (ifr_hwaddr doesn't exist on Solaris).

- The implementation of guest_get_network_stats is Linux-specific, so
  hide it under #ifdef CONFIG_LINUX. On non-Linux, we just won't
  provide network interface stats.

Signed-off-by: Andrew Deason <adeason@sinenomine.net>
Reviewed-by: Michal Privoznik <mprivozn@redhat.com>
Message-Id: <20220426195526.7699-4-adeason@sinenomine.net>