util: A generic function to convert error to string for windows.

More users will be added in an upcoming commit.

Signed-off-by: Gurucharan Shetty <gshetty@nicira.com>
Acked-by: Ben Pfaff <blp@nicira.com>
upcall: Avoid divide-by-zero calculating flow limit

Signed-off-by: Joe Stringer <joestringer@nicira.com>
Acked-by: Ethan Jackson <ethan@nicira.com>