libvirt.test: Add some more fail information

1.Skip test if qga is not respond
2.Show the bug link for guest-shutdown
Signed-off-by: Li Yang <liyang.fnst@cn.fujitsu.com>
libvirt.test: Add fsfreeze command for qemu-agent-command test

Signed-off-by: Li Yang <liyang.fnst@cn.fujitsu.com>