virDomainDiskDefValidateSourceChainOne: Fix validation of 'data-file' nesting

virDomainDiskDefValidateSourceChainOne: Fix validation of 'data-file' nesting

As the 'dataStore' is internally represented as a virStorageSource
object it has provisions for nesting which is not supported.

When I've reviewed and modified the commit adding data file parsing
support I've added code that was supposed to reject any 'backingStore'
and 'dataStore' structures nested in a source of a 'dataStore'.

Unfortunately the check was broken as one of the terms checked the
presence of parent's 'backingStore' instead of the nesting.

Fix it and add tests.

Fixes: b3171cf8da3
Resolves: https://issues.redhat.com/browse/RHEL-85320
Signed-off-by: Peter Krempa <pkrempa@redhat.com>
Reviewed-by: Jiri Denemark <jdenemar@redhat.com>