datapath-windows: refactor code to setup dump start state

Per review comment, in this patch we refactor the code to create a
OvsSetupDumpStart() which can be leveraged by dump functions in the
future. I have not refactored the code that continues the dump
operation primarily since it is not final yet. Once the netlink set
APIs are in place, we can refactor that too.

Signed-off-by: Nithin Raju <nithin@vmware.com>
Acked-by: Ankur Sharma <ankursharma@vmware.com>
Acked-by: Samuel Ghinet <sghinet@cloudbasesolutions.com>
Signed-off-by: Ben Pfaff <blp@nicira.com>