Description
nwfilter-edit should give error when modify the value to inadmissibility, not return successfully as delete the line which including inadmissible value.

Version:
libvirt-0.10.2-18.el6_4.9.x86_64

How reproducible:
100%

Steps to Reproduce:
1. # vim disallow-arp.xml
<filter name='disallow-arp' chain='arp'>
  <rule action='drop' direction='inout' priority='500'/>
</filter>

2. # virsh nwfilter-define disallow-arp.xml
Network filter disallow-arp defined from disallow-arp.xml

3. # virsh nwfilter-dumpxml disallow-arp
<filter name='disallow-arp' chain='arp' priority='-500'>
  <uuid>2f737f78-779c-4b2c-b223-b67b90765afb</uuid>
  <rule action='drop' direction='inout' priority='500'/>
</filter>

4. Use nwfilter-edit "rule action='drop'" to “rule action='dropaaa'” and save:
# virsh nwfilter-edit disallow-arp
Network filter disallow-arp XML configuration edited.

5. the rule line will disappear:
# virsh nwfilter-dumpxml disallow-arp
<filter name='disallow-arp' chain='arp' priority='-500'>
  <uuid>e088d644-6e76-4827-a7ea-ba1ea924dd1c</uuid>
</filter>

6. Also for nwfilter-define works like that.

Actual results：
As description.

Expected results:
nwfilter-edit should give error when modify the value to inadmissibility, not return successfully as delete the line which including inadmissible value.
It should judge 'rule action' only support 'drop',' reject','accept','return','continue' and "direction" only support 'in','out' and 'inout' and give error info but just delete the rule if there are not those value.

Additional info:
For nwfilter-define, if there is some inadmissible value like "dropaaa" in xml, define will succeed, but there is no <rule> element in that nwfilter. Also need to fix.