target-s390x: make cpu-qom.h not target specific

target-s390x: make cpu-qom.h not target specific

Make S390XCPU an opaque type within cpu-qom.h, and move all definitions
of private methods, as well as all type definitions that require knowledge
of the layout to cpu.h.  This helps making files independent of NEED_CPU_H
if they only need to pass around CPU pointers.

Acked-by: Cornelia Huck <[email protected]>
Signed-off-by: Paolo Bonzini <[email protected]>