conf: node_device: Refactor 'virNodeDeviceCapVPDParseCustomFields' to fix error reporting

conf: node_device: Refactor 'virNodeDeviceCapVPDParseCustomFields' to fix error reporting

The errors raised in virNodeDeviceCapVPDParseCustomFields were actually
ignored by continuing the parse rather than raised.

Rather than just replace 'continue' by 'return -1' this patch refactors
the whole parser to simplify it as well as report reasonable errors.

Parsing of individual fields is done without XPath and is extracted into
a common helper.

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