util: rename virNetFilterAction to iptablesAction, and add VIR_ENUM_DECL/IMPL

util: rename virNetFilterAction to iptablesAction, and add VIR_ENUM_DECL/IMPL

I had originally named these as VIR_NETFILTER_* because I assumed the
same enum would eventually be used by our nftables backend as well as
iptables. But it turns out that in most cases it's not possible to
delete an nftables rule, so we just never used the enum anyway, so
this patch is renaming the values to IPTABLES_ACTION_*, and taking
advantage of the newly defined (via VIR_ENUM_DECL/IMPL)
iptablesActionTypeToString() to replace all the ternary operators used
to translate the enum into a string for the iptables commandline with
iptablesActionTypeToString().

Signed-off-by: Laine Stump <laine@redhat.com>
Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>