Added explicit typecasting to libuv plugin to avoid msvc warnings.

libuv defines uv_buf_t's len field differently on windows then on
other platforms which causes a miss-match when building on
windows.

Change-Id: I00c65c8b84f47b3e64a3e01154e637e641579465
Reviewed-on: http://review.couchbase.org/29758
Reviewed-by: Brett Lawson <brett19@gmail.com>
Tested-by: Brett Lawson <brett19@gmail.com>