qemu: domain: Clear startup policy for dropped removable media

When a source image is dropped when missing due to startup policy the
policy needs to be cleared since it was relevant only for the given
storage source. New sources need to update it if needed.

src/qemu/qemu_domain.c

@@ -4341,6 +4341,8 @@ qemuDomainCheckRemoveOptionalDisk(virQEMUDriverPtr driver,
                                                    disk->info.alias,
                                                    VIR_DOMAIN_EVENT_DISK_CHANGE_MISSING_ON_START);
         ignore_value(virDomainDiskSetSource(disk, NULL));
+        /* keeping the old startup policy would be invalid for new images */
+        disk->startupPolicy = VIR_DOMAIN_STARTUP_POLICY_DEFAULT;
     } else {
         event = virDomainEventDiskChangeNewFromObj(vm, src, NULL,
                                                    disk->info.alias,