VDPDataStore and VDPRegViewer code repetition

[pvmm]

	int total = TOTAL_SIZE - !registerLatchAvailable - !paletteLatchAvailable - !dataLatchAvailable;
	new SimpleHexRequest(req, total, regs, *this);

This code block is largely repeated between VDPDataStore and VDPRegViewer. Can we (in a separate commit) reduce this duplication? Ideally store/query this information only in a single location.

Originally posted by @m9710797 in https://github.com/openMSX/debugger/pull/157#discussion_r1096511840