libvirt_vm: give a choice on starting serial console automatically.

In start()&create() of VM class, they will start
serial console automatic when vm is started.
It will prevent other test from using console.
So give a choice on starting serial console automatically.

Signed-off-by: Yu Mingfei <yumingfei@cn.fujitsu.com>
virt-libvirt: Add test for virsh console.

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