qemuDomainUpdateDeviceConfig: Allow startupPolicy update

https://bugzilla.redhat.com/show_bug.cgi?id=1159219

Users might want to update startupPolicy via the
virDomainUpdateDeviceFlags API too. This patch
implements the feature on config layer.
Signed-off-by: NMichal Privoznik <mprivozn@redhat.com>

src/qemu/qemu_driver.c

@@ -7437,6 +7437,7 @@ qemuDomainUpdateDeviceConfig(virQEMUCapsPtr qemuCaps,
         if (disk->src->format)
             orig->src->format = disk->src->format;
         disk->src->path = NULL;
+        orig->startupPolicy = disk->startupPolicy;
         break;
 
     case VIR_DOMAIN_DEVICE_NET: