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

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

Make MIPSCPU 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.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>