ovs-ctl.in: Restore flows with vswitchd packet processing disabled.

It has been observed that when we upgrade openvswitch with
thousands of openflow flows already established and there is heavy
traffic going through the switch, restoring flows can take a very
long time (8 minutes in one use case) with CPU running at 100%.
This can make the upgrade very expensive.

This commit starts vswitchd by asking it to wait till flow restore is
finished. Once the flows are restored, it resumes normal operations.

Bug #16086.
Signed-off-by: Gurucharan Shetty <gshetty@nicira.com>