conf: backup: Fix logic for generating default backup filenames

conf: backup: Fix logic for generating default backup filenames

If the 'disk->store' property is already allocated which happens e.g.
when the disk is described by the backup XML but the optional filename
is not filled in 'virDomainBackupDefAssignStore' would not fill in the
default location.

Fix the logic to do it also if a 'virStorageSource' categorizes as
empty.

Signed-off-by: Peter Krempa <pkrempa@redhat.com>
Reviewed-by: Ján Tomko <jtomko@redhat.com>