datapath-windows: add support for GET_DP command to dump datpaths

datapath-windows: add support for GET_DP command to dump datpaths

In this patch, we add support for the GET_DP netlink command to dump
the datpaaths. The userspace workflow to get this to work is the same
as on Linux. dpif-linux.c initiates a dump start by writing a netlink
message, and after that continues to read data from the kernel while
the kernel has data. The state is maintained in the kernel, and not in
userspace. This approach was taken since there was not great benefit
of maintaining state in userspace, and also to avoid userspace changes
specific to Windows.

This hopefully serves as a template to base the other dump commands on.

validation:
- With a hacked up dpif-linux.c to work on Windows,
  dpif_linux_enumerate() successfully enumerated the datapaths in the
  kernel.

Signed-off-by: Nithin Raju <nithin@vmware.com>
Signed-off-by: Ankur Sharma <ankursharma@vmware.com>
Acked-by: Ankur Sharma <ankursharma@vmware.com>
Acked-by: Saurabh Shah <ssaurabh@vmware.com>
Signed-off-by: Ben Pfaff <blp@nicira.com>