conf: Combine if condition in virDomainObjUpdateModificationImpact

conf: Combine if condition in virDomainObjUpdateModificationImpact

Prior to commit id '3d021381' virDomainObjUpdateModificationImpact was
part of virDomainLiveConfigHelperMethod and the *flags if condition
VIR_DOMAIN_AFFECT_CONFIG checked the ->persistent boolean and made the
virDomainObjGetPersistentDef call.

Since the functions were split the ->persistent check is all that remained
and thus could be combined into one if statement.

Signed-off-by: Nikolay Shirokovskiy <nshirokovskiy@virtuozzo.com>
Signed-off-by: John Ferlan <jferlan@redhat.com>