virt:Fix a bug about help_command()

Signed-off-by: Li Yang <liyang.fnst@cn.fujitsu.com>
virt: Add virsh function for virsh help

Signed-off-by: Li Yang <liyang.fnst@cn.fujitsu.com>
libvirt: Add test module for virsh help

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