nbd: Don't use *_to_cpup() functions

nbd: Don't use *_to_cpup() functions

The *_to_cpup() functions are not very useful, as they simply do
a pointer dereference and then a *_to_cpu(). Instead use either:
 * ld*_*_p(), if the data is at an address that might not be
   correctly aligned for the load
 * a local dereference and *_to_cpu(), if the pointer is
   the correct type and known to be correctly aligned

Signed-off-by: Peter Maydell <[email protected]>
Message-Id: <1465570836[email protected]>
Signed-off-by: Paolo Bonzini <[email protected]>