xen/privcmd: fix MMAP_RESOURCE ioctl to copy out results

The current definition for the MMAP_RESOURCE ioctl was wrong as it
didn't copy back the result to the caller. Fix the definition and also
remove the bogus attempt to copy the result in the implementation.

Note such copy back is only needed when querying the size of a
resource.

Sponsored by: Citrix Systems R&D