memory: call log_start after region_add

It might be confusing for some listener implementations that implement
both, region_add and log_start (e.g. KVM) if we call log_start before an
actual region was added using region_add.

This makes current KVM code trigger an assertion
("kvm_section_update_flags: error finding slot"). So let's just reverse
the order instead of tolerating log_start on yet unknown regions.

Reported-by: Thomas Huth <thuth@redhat.com>
Signed-off-by: David Hildenbrand <david@redhat.com>
Message-Id: <20171016144302.24284-2-david@redhat.com>
Tested-by: Joe Clifford <joeclifford@gmail.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>