qemu: snapshot: Skip empty drives with internal snapshots

The code that validates whether an internal snapshot is possible would
reject an empty but not-readonly drive. Since floppies can have this
property, add a check for emptiness.

(cherry picked from commit eee3b4b9)

src/qemu/qemu_driver.c

@@ -13831,7 +13831,8 @@ qemuDomainSnapshotPrepare(virConnectPtr conn,
 
         case VIR_DOMAIN_SNAPSHOT_LOCATION_NONE:
             /* Remember seeing a disk that has snapshot disabled */
-            if (!dom_disk->src->readonly)
+            if (!virStorageSourceIsEmpty(dom_disk->src) &&
+                !dom_disk->src->readonly)
                 forbid_internal = true;
             break;