cpu: make cpu-qom.h only include-able from cpu.h

cpu: make cpu-qom.h only include-able from cpu.h

Make cpu-qom.h so that it is only included from cpu.h.  Then there
is no need for it to include cpu.h again.

Later we will make cpu-qom.h target independent and we will _want_
to include it from elsewhere, but for now reduce the number of cases
to handle.

Reviewed-by: Alex Bennée <alex.bennee@linaro.org>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>