libvirt_bench: make ttcp_from_guest_to_host works for vms rather than only for main_vm.

Signed-off-by: Dongsheng Yang <yangds.fnst@cn.fujitsu.com>
Merge pull request #194 from yangdongsheng/ttcp

libvirt_bench: make ttcp_from_guest_to_host works for vms rather than on...