qemuDomainDeviceDefValidateDisk: Reorder some checks

I find this function more readable if checks for passed storage
source are done first and backing chain is done last. Mixing them
together does not hurt, but is less readable.
Signed-off-by: NMichal Privoznik <mprivozn@redhat.com>
ACKed-by: NPeter Krempa <pkrempa@redhat.com>

src/qemu/qemu_domain.c

@@ -5524,11 +5524,6 @@ qemuDomainDeviceDefValidateDisk(const virDomainDiskDef *disk,
         return -1;
     }
 
-    for (n = disk->src; virStorageSourceIsBacking(n); n = n->backingStore) {
-        if (qemuDomainValidateStorageSource(n, qemuCaps) < 0)
-            return -1;
-    }
-
     if (disk->device == VIR_DOMAIN_DISK_DEVICE_CDROM &&
         disk->bus == VIR_DOMAIN_DISK_BUS_VIRTIO) {
         virReportError(VIR_ERR_CONFIG_UNSUPPORTED,
@@ -5537,6 +5532,11 @@ qemuDomainDeviceDefValidateDisk(const virDomainDiskDef *disk,
         return -1;
     }
 
+    for (n = disk->src; virStorageSourceIsBacking(n); n = n->backingStore) {
+        if (qemuDomainValidateStorageSource(n, qemuCaps) < 0)
+            return -1;
+    }
+
     return 0;
 }