migration/rdma: Safely convert control types

control_desc[] is an array of strings that correspond to a
series of message types; they're used only for error messages, but if
the message type is seriously broken then we could go off the end of
the array.

Convert the array to a function control_desc() that bound checks.

Signed-off-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
Reviewed-by: Peter Xu <peterx@redhat.com>
Reviewed-by: Juan Quintela <quintela@redhat.com>
Message-Id: <20170717110936.23314-6-dgilbert@redhat.com>
Signed-off-by: Juan Quintela <quintela@redhat.com>
