ovs-lib: Add process name checking when start ovs service

Avoids "false positive" for the service already running due to pid reuse.

Signed-off-by: Chunhe Li <lichunhe@huawei.com>
Signed-off-by: Ben Pfaff <blp@nicira.com>
Reported-by: Wuyunfei <wuyunfei@huawei.com>