virshFindDisk: Sanitize use of 'tmp' variable

virshFindDisk: Sanitize use of 'tmp' variable

The return value of virXMLPropString was assigned into 'tmp' multiple
times and to prevent static analyzers moaning about a potential leak a
short-circuited if logic or was used.

Replace the code by having a helper variable for each possibility and
also replace the for-loop to iterate elements.

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