qemu: Clean up snapshot retrieval to use the new helper

Two other places were left with the old code to look up snapshots.
Change them to use the snapshot lookup helper.

src/qemu/qemu_driver.c

@@ -11832,13 +11832,8 @@ static int qemuDomainRevertToSnapshot(virDomainSnapshotPtr snapshot,
         goto cleanup;
     }
 
-    snap = virDomainSnapshotFindByName(vm->snapshots, snapshot->name);
-    if (!snap) {
-        virReportError(VIR_ERR_NO_DOMAIN_SNAPSHOT,
-                       _("no domain snapshot with matching name '%s'"),
-                       snapshot->name);
+    if (!(snap = qemuSnapObjFromSnapshot(vm, snapshot)))
         goto cleanup;
-    }
 
     if (!vm->persistent &&
         snap->def->state != VIR_DOMAIN_RUNNING &&
@@ -12200,13 +12195,8 @@ static int qemuDomainSnapshotDelete(virDomainSnapshotPtr snapshot,
         goto cleanup;
     }
 
-    snap = virDomainSnapshotFindByName(vm->snapshots, snapshot->name);
-    if (!snap) {
-        virReportError(VIR_ERR_NO_DOMAIN_SNAPSHOT,
-                       _("no domain snapshot with matching name '%s'"),
-                       snapshot->name);
+    if (!(snap = qemuSnapObjFromSnapshot(vm, snapshot)))
         goto cleanup;
-    }
 
     if (!(flags & VIR_DOMAIN_SNAPSHOT_DELETE_METADATA_ONLY)) {
         if (!(flags & VIR_DOMAIN_SNAPSHOT_DELETE_CHILDREN_ONLY) &&