IB/uverbs: Support for query QP from userspace

Add support to uverbs to handle querying userspace QPs (queue pairs),
including adding an ABI for marshalling requests and responses.  The
kernel midlayer already has the underlying ib_query_qp() function.

Signed-off-by: Dotan Barak <dotanb@mellanox.co.il>
Signed-off-by: Roland Dreier <rolandd@cisco.com>