util: add new "tc" layer for virFirewallCmd objects

util: add new "tc" layer for virFirewallCmd objects

If the layer of a virFirewallCmd is "tc", then the "tc" utility will
be executed using the arguments that had been added to the
virFirewallCmd

tc layer doesn't support auto-rollback command creation (any rollback
needs to be added manually with virFirewallAddRollbackCmd()), and also
tc layer isn't supported by the iptables backend (it would have been
straightforward to add, but the iptables backend doesn't need it, and
I didn't want to take the chance of causing a regression in that
code for no good reason).

Signed-off-by: Laine Stump <laine@redhat.com>
Signed-off-by: Michal Privoznik <mprivozn@redhat.com>
Reviewed-by: Michal Privoznik <mprivozn@redhat.com>