snapshot: implement getparent in qemu

First hypervisor implementation of the new API.
Allows 'virsh snapshot-list --tree' to be more efficient.

* src/qemu/qemu_driver.c (qemuDomainSnapshotGetParent): New
function.

src/qemu/qemu_driver.c

@@ -9450,6 +9450,51 @@ cleanup:
     return ret;
 }
 
+static virDomainSnapshotPtr
+qemuDomainSnapshotGetParent(virDomainSnapshotPtr snapshot,
+                            unsigned int flags)
+{
+    struct qemud_driver *driver = snapshot->domain->conn->privateData;
+    virDomainObjPtr vm;
+    virDomainSnapshotObjPtr snap = NULL;
+    virDomainSnapshotPtr parent = NULL;
+
+    virCheckFlags(0, NULL);
+
+    qemuDriverLock(driver);
+    vm = virDomainFindByUUID(&driver->domains, snapshot->domain->uuid);
+    if (!vm) {
+        char uuidstr[VIR_UUID_STRING_BUFLEN];
+        virUUIDFormat(snapshot->domain->uuid, uuidstr);
+        qemuReportError(VIR_ERR_NO_DOMAIN,
+                        _("no domain with matching uuid '%s'"), uuidstr);
+        goto cleanup;
+    }
+
+    snap = virDomainSnapshotFindByName(&vm->snapshots, snapshot->name);
+    if (!snap) {
+        qemuReportError(VIR_ERR_NO_DOMAIN_SNAPSHOT,
+                        _("no domain snapshot with matching name '%s'"),
+                        snapshot->name);
+        goto cleanup;
+    }
+
+    if (!snap->def->parent) {
+        qemuReportError(VIR_ERR_NO_DOMAIN_SNAPSHOT,
+                        _("snapshot '%s' does not have a parent"),
+                        snap->def->name);
+        goto cleanup;
+    }
+
+    parent = virGetDomainSnapshot(snapshot->domain, snap->def->parent);
+
+cleanup:
+    if (vm)
+        virDomainObjUnlock(vm);
+    qemuDriverUnlock(driver);
+    return parent;
+}
+
 static virDomainSnapshotPtr qemuDomainSnapshotCurrent(virDomainPtr domain,
                                                       unsigned int flags)
 {
@@ -10431,6 +10476,7 @@ static virDriver qemuDriver = {
     .domainSnapshotListNames = qemuDomainSnapshotListNames, /* 0.8.0 */
     .domainSnapshotLookupByName = qemuDomainSnapshotLookupByName, /* 0.8.0 */
     .domainHasCurrentSnapshot = qemuDomainHasCurrentSnapshot, /* 0.8.0 */
+    .domainSnapshotGetParent = qemuDomainSnapshotGetParent, /* 0.9.7 */
     .domainSnapshotCurrent = qemuDomainSnapshotCurrent, /* 0.8.0 */
     .domainRevertToSnapshot = qemuDomainRevertToSnapshot, /* 0.8.0 */
     .domainSnapshotDelete = qemuDomainSnapshotDelete, /* 0.8.0 */