virPCIVPDParseVPDLargeResourceFields: Refactor return logic

virPCIVPDParseVPDLargeResourceFields: Refactor return logic

Rewrite the conditions after exiting the parser so that they are easier
to understand. This partially decreases the granularity of "error"
messages as they are not strictly necessary albeit for debugging.

As it was already observed in this code the logic itself often does
something else than the comment claims, thus the code logic is
preserved.

Changes:
 - any case when not all data was processed is aggregated together and
   gets a common "error" message
 - absence of 'checksum' field is checked separately
 - helper variables are removed as they are no longer needed

Signed-off-by: Peter Krempa <pkrempa@redhat.com>
Reviewed-by: Ján Tomko <jtomko@redhat.com>