find_ram_offset: Add comments and tracing

Add some comments so I can understand the various nested loops.
Add some tracing so I can see what they're doing.

Signed-off-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
Message-Id: <20180105170138.23357-2-dgilbert@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
