https://bugzilla.redhat.com/show_bug.cgi?id=1019053

When we migrate vms concurrently, there's a chance that libvirtd on
destination assigns the same port for different migrations, which will
lead to migration failure during prepare phase on destination. So we use
virPortAllocator here to solve the problem.
Signed-off-by: NWang Yufei <james.wangyufei@huawei.com>
Signed-off-by: NJiri Denemark <jdenemar@redhat.com>