Libvirt: Virsh net define undefine

This is patchset for virsh net-define/undefine test.

Change from v1:
1.Add virsh net-undefine testcases.
2.Add transaction test between net-define and net-undefine.

issue: 
https://github.com/autotest/virt-test/issues/208
https://github.com/autotest/virt-test/issues/237