cpu: refactor cpu_address_space_init()

Normally we create an address space for that CPU and pass that address
space into the function.  Let's just do it inside to unify address space
creations.  It'll simplify my next patch to rename those address spaces.

Signed-off-by: Peter Xu <peterx@redhat.com>
Message-Id: <20171123092333.16085-3-peterx@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>