Commit 27e81517 set the payload size to 256 KB, which is
actually the max packet size, including the size of the header.

Reduce this by VIR_NET_MESSAGE_HEADER_MAX (24) and set
VIR_NET_MESSAGE_LEGACY_PAYLOAD_MAX to 262120, which was the original
value before increasing the limit in commit eb635de1.

(cherry picked from commit 609eb987)