conf: Add new helpers to resolve virDomainModificationImpact to domain defs

conf: Add new helpers to resolve virDomainModificationImpact to domain defs

virDomainLiveConfigHelperMethod that is used for this job now does
modify the flags but still requires the callers to extract the correct
definition objects.

In addition coverity and other static analyzers are usually unhappy as
they don't grasp the fact that @flags are upadted according to the
correct def to be present.

To work this issue around and simplify the calling chain let's add a new
helper that will work only on drivers that always copy the persistent
def to a transient at start of a vm. This will allow to drop a few
arguments. The new function syntax will also fill two definition
pointers rather than modifying the @flags parameter.