Push virNWFilterRuleInstPtr out of (eb|ip)tablesCreateRuleInstance

Push virNWFilterRuleInstPtr out of (eb|ip)tablesCreateRuleInstance

Later refactoring will change use of the virNWFilterRuleInstPtr struct.
Prepare for this by pushing use of the virNWFilterRuleInstPtr parameter
out of the ebtablesCreateRuleInstance and iptablesCreateRuleInstance
methods. Instead they simply string(s) with the constructed rule data.
The ebiptablesCreateRuleInstance method will make use of the
virNWFilterRuleInstPtr struct instead.

Signed-off-by: Daniel P. Berrange <berrange@redhat.com>