qemuDomainUpdateDeviceConfig: Allow startupPolicy update, yet again

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

So, in 11e058ca I've tried to make UpdateDevice update
startupPolicy too. And it worked well until somebody came around
and pushed d0dc6c03 which accidentally removed my
contribution. Redo my commit.
Signed-off-by: NMichal Privoznik <mprivozn@redhat.com>

src/qemu/qemu_driver.c

@@ -8338,6 +8338,7 @@ qemuDomainUpdateDeviceConfig(virQEMUCapsPtr qemuCaps,
          * We allow updating src/type//driverType/cachemode/
          */
         orig->cachemode = disk->cachemode;
+        orig->startupPolicy = disk->startupPolicy;
 
         virStorageSourceFree(orig->src);
         orig->src = disk->src;