libvirt_vm: Add exists() for libvirt VM class.

Signed-off-by: Yu Mingfei <yumingfei@cn.fujitsu.com>
virsh: Add migrate_setmaxdowntime method.

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

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