qemu: snapshot: Fix incorrect disk type for auto-generated disks

When changing the parsing and formatting functions in commit
43f2ccdc I forgot to update the qemu
disk alignment function for snapshots that automatically adds snapshot
configs for disks that were not mentioned in the XML. The function
allocated a new disk snapshot definition but did not correctly
initialize the snapshot disk source type variable. This resulted into
the disks considered as block devices and invalid XML was generated.

Reported by John Ferlan.

src/conf/snapshot_conf.c

@@ -532,6 +532,7 @@ virDomainSnapshotAlignDisks(virDomainSnapshotDefPtr def,
             goto cleanup;
         disk->index = i;
         disk->snapshot = def->dom->disks[i]->snapshot;
+        disk->type = -1;
         if (!disk->snapshot)
             disk->snapshot = default_snapshot;
     }
@@ -549,6 +550,15 @@ virDomainSnapshotAlignDisks(virDomainSnapshotDefPtr def,
             const char *tmp;
             struct stat sb;
 
+            if (disk->type != VIR_DOMAIN_DISK_TYPE_FILE &&
+                disk->type != -1) {
+                virReportError(VIR_ERR_CONFIG_UNSUPPORTED,
+                               _("cannot generate external snapshot name "
+                                 "for disk '%s' on a '%s' device"),
+                               disk->name, virDomainDiskTypeToString(disk->type));
+                goto cleanup;
+            }
+
             if (!original) {
                 virReportError(VIR_ERR_CONFIG_UNSUPPORTED,
                                _("cannot generate external snapshot name "