virttest.virsh: Add virsh iface-{begin, commit, rollback} commands function wrapper

Signed-off-by: Alex Jia <ajia@redhat.com>
libvirt.tests: Add host network transaction test cases

Signed-off-by: Alex Jia <ajia@redhat.com>
libvirt.tests: Fix iface_trans test for latest next.

* Move test module to tests/src/virsh_cmd/interface
* Remove useless blackslashes.

Signed-off-by: Yu Mingfei <yumingfei@cn.fujitsu.com>