conf: def: Avoid unnecessary allocation of 'perf' events definition

conf: def: Avoid unnecessary allocation of 'perf' events definition

Some code paths already assume that it is allocated since it was always
allocated by virDomainPerfDefParseXML. Make it member of virDomainDef
directly so that we don't have to allocate it all the time.

This fixes crash when attempting to connect to an existing process via
virDomainQemuAttach since we would not allocate it in that code path.

Resolves: https://bugzilla.redhat.com/show_bug.cgi?id=1350688