virDomainDiskTranslateSourcePool: Fix check of 'startupPolicy' definition

virDomainDiskTranslateSourcePool: Fix check of 'startupPolicy' definition

The check was historically done only for _TYPE_VOLUME disks, but
refactors to allow _TYPE_VOLUME disks in the backing chain caused a
regression where we'd reject startupPolicy also for _TYPE_BLOCK disks
which historically worked well.

Fix it by using the 'virDomainDiskDefValidateStartupPolicy' helper and
use it only when the top level image is a _TYPE_VOLUME as in other cases
it was already validated. This also allows _TYPE_BLOCK volumes to use
startup policy.

Fixes: 37f01262eed9f37dd5eb7de8b83edd2fea741054
Resolves: https://bugzilla.redhat.com/show_bug.cgi?id=2095758
Signed-off-by: Peter Krempa <pkrempa@redhat.com>
Reviewed-by: Ján Tomko <jtomko@redhat.com>