Description of problem:
Libvirt report incorrect error message when parsing invalid value of CTRL_IP_LEARNING in nwfilter

Version-Release number of selected component (if applicable):
libvirt-1.1.1-25.el7.x86_64
qemu-kvm-rhev-1.5.3-50.el7.x86_64
kernel-3.10.0-97.el7.x86_64

How reproducible:
100%

Steps to Reproduce:
1. Set invalid value to CTRL_IP_LEARNING, Valid values are any, dhcp, or none.
[root@intel-e5530-8-2 ~]# virsh dumpxml r7 | grep interface -A8
    <interface type='network'>
      <mac address='52:54:00:7f:44:cb'/>
      <source network='default'/>
      <model type='rtl8139'/>
      <filterref filter='clean-traffic'>
        <parameter name='CTRL_IP_LEARNING' value='jiahu'/>                
      </filterref>
      <address type='pci' domain='0x0000' bus='0x00' slot='0x03' function='0x0'/>
    </interface>
...

2. Start the domain.
[root@intel-e5530-8-2 ~]# virsh start r7
error: Failed to start domain r7
error: internal error: Could not find value for variable 'IP'

Actual results:
As shown above, after checking the codes, lack of "goto err_exit;" after the "virReportError" sentence in virNWFilterInstantiate function in nwfilter/nwfilter_gentech_driver.c

 710             } else {
 711                 rc = -1;
 712                 virReportError(VIR_ERR_PARSE_FAILED,
 713                                _("filter '%s' "
 714                                  "learning value '%s' invalid."),
 715                                filter->name, learning);
 716             }
 717         } else

Expected results:
Rebuild libvirt with "goto err_exit;" line after src/nwfilter/nwfilter_gentech_driver.c:715， below message is expected results.

[root@intel-3323-24-1 src]# rpm -q libvirt
libvirt-1.1.1-25.el7_jiahu_testing.x86_64
[root@intel-3323-24-1 src]#
[root@intel-3323-24-1 src]# virsh dumpxml mig1| grep interface -A8
    <interface type='network'>
      <mac address='52:54:00:b7:e7:d8'/>
      <source network='default'/>
      <model type='rtl8139'/>
      <filterref filter='clean-traffic'>
        <parameter name='CTRL_IP_LEARNING' value='jiahu'/>
      </filterref>
      <address type='pci' domain='0x0000' bus='0x00' slot='0x03' function='0x0'/>
    </interface>
...

[root@intel-3323-24-1 src]# virsh start mig1
error: Failed to start domain mig1
error: failed to parse configuration file filter 'clean-traffic' learning value 'jiahu' invalid.