qemu: migration: Use virStorageSourceIsEmpty in qemuMigrateDisk

Use the proper check whether a disk is empty.

src/qemu/qemu_migration.c

@@ -415,7 +415,7 @@ qemuMigrateDisk(virDomainDiskDef const *disk,
     /* Default is to migrate only non-shared non-readonly disks
      * with source */
     return !disk->src->shared && !disk->src->readonly &&
-           virDomainDiskGetSource(disk);
+           !virStorageSourceIsEmpty(disk->src);
 }