cpus: initialize cpu->memory_dispatch

This fixes a NULL pointer dereference in s390x-softmmu.

On pretty much all other architectures, creating an MMIO region calls
cpu_reload_memory_map.  On s390, however, there are no MMIO regions
and everything is done via hypercalls.

Fixes: 9d82b5a792236db31a75b9db5c93af69ac07c7c5
Reported-by: Christian Borntraeger <borntraeger@de.ibm.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
