The portmapper (on tcp/111 and udp/111) does exactly that: It allows the server to allocate somewhat-arbitrary TCP and UDP ports (i.e. not well-known port numbers) to provide certain RPC (Remote Procedure Call) services, and provides a lookup service so that RPC clients can determine how to connect to each service – see man 8 portmap for more information. It's up to the RPC server (in this case, your QNAP) to determine which ports should be used for each service.
In this case, ESXi is contacting the portmapper service on your QNAP's udp/111 or tcp/111 and requesting some RPC service. The QNAP is responding to each client with a message saying "hey, sure, you can get to that service by connecting back to me on tcp/42445" (or whatever), so that is what your ESXi box does. You may be able to use tools like rpcinfo to find out which proto/port on the QNAP is servicing each RPC program/version. That might help with understanding the nature and origin of these connections...
Cheers,
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Darius