qemu.qemu_vm: support vhostfd in netdev.

vhostfd only works when vhost on.
add enable_vhost parameter. set default value to 'yes'
Signed-off-by: Feng Yang <fyang@redhat.com>
utils_libvirtd: if libvirt is not availible in host, No error, but a warning.

Signed-off-by: yangdongsheng <yangds.fnst@cn.fujitsu.com>
virt: modify libvirtd_status() to libvirtd_is_running()

Signed-off-by: Li Yang <liyang.fnst@cn.fujitsu.com>
libvirt.tests: Add test module of migration with stress.

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