qemu: blockcopy: Don't remove existing disk mirror info

When creating a new disk mirror the new struct is stored in a separate
variable until everything went well. The removed hunk would actually
remove existing mirror information for example when the api would be run
if a mirror still exists.

src/qemu/qemu_driver.c

@@ -15410,10 +15410,6 @@ qemuDomainBlockCopy(virDomainObjPtr vm,
  endjob:
     if (need_unlink && unlink(dest))
         VIR_WARN("unable to unlink just-created %s", dest);
-    if (ret < 0 && disk) {
-        virStorageSourceFree(disk->mirror);
-        disk->mirror = NULL;
-    }
     virStorageSourceFree(mirror);
     if (!qemuDomainObjEndJob(driver, vm))
         vm = NULL;