util: better error message after failure to initialize firewall backend

util: better error message after failure to initialize firewall backend

If the firewalld backend wasn't available and libvirt decides to try
setting up a "direct" backend, it checks for the presence of iptables,
ip6tables, and ebtables. If they are not found, a message like this is logged:

  error : virFirewallValidateBackend:193 : direct firewall backend
          requested, but /usr/sbin/ip6tables is not available:
          No such file or directory

But then at a later time if an attempt is made to use the virFirewall
API, failure will be indicated with:

  error : virFirewallApply:936 : out of memory

This patch changes virFirewallApply to first check if a firewall
backend hadn't been successfully setup, and logs a slightly more
informative message in that case:

  error : virFirewallApply:940 : internal error:
          Failed to initialize a valid firewall backend

This resolves: https://bugzilla.redhat.com/show_bug.cgi?id=1223876