Merge xgi_mem_req and xgi_mem_alloc into a single type.

Merge xgi_mem_req and xgi_mem_alloc into a single type.

These two structures were used as the request and reply for certain
ioctls.  Having a different type for an ioctl's input and output is
just wierd.  In addition, each structure contained fields (e.g., pid)
that had no business being there.

This change requires updates to user-space.