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

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

Make XtensaCPU 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.  Conversely, move all definitions needed to
define a class to cpu-qom.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>
Signed-off-by: Max Filippov <jcmvbkbc@gmail.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>