virttest.utils_libvirtd: support remotely control libvirtd

Signed-off-by: Alex Jia <chuanchang.jia@gmail.com>
virttest.utils_test.libvirt: add new test utilities

Signed-off-by: Alex Jia <chuanchang.jia@gmail.com>
Merge pull request #1729 from chuanchang/add_utils_test_for_libvirt

virttest.utils_test.libvirt: add new test utilities