vz: add validation callbacks

vz: add validation callbacks

This patch fixes a bug which occurs when we check a bus and unit number
for a new attached disk. We should do this check in ValidadionCallback,
not in PostParse callback. Because in PostParse we have not initialized
disk->info.addr.drive struct yet.
Move part of code from domainPostParseCallback to domainValidateCallback
and part from devicesPostParseCallback to deviceValidateCallback.
PostParse callbacks are for modification data.
ValidateCallbacks are only for checks.