conf: cpu: Refactor parsing of vendor_id and fallback attributes

conf: cpu: Refactor parsing of vendor_id and fallback attributes

This patch simplifies the code that parses the fallback and vendor_id
attributes from the domain xml cpu definition.

Changes done:
- free temp variables in the cleanup section instead of local use
- remove checking for presence of the attribute to directly getting the
value (saving call to virXPathBoolean)
- replace loop used to check for ',' in the vendor_id string with strchr