snapshot: support extra state in snapshots

In order to distinguish disk snapshots from system checkpoints, a
new state value that is only valid for snapshots is helpful.

* include/libvirt/libvirt.h.in (VIR_DOMAIN_LAST): New placeholder.
* src/conf/domain_conf.h (virDomainSnapshotState): New enum mapping.
(VIR_DOMAIN_DISK_SNAPSHOT): New internal enum value.
* src/conf/domain_conf.c (virDomainState): Use placeholder.
(virDomainSnapshotState): Extend mapping by one for use in snapshot.
(virDomainSnapshotDefParseString, virDomainSnapshotDefFormat):
Handle new state.
(virDomainObjSetState, virDomainStateReasonToString)
(virDomainStateReasonFromString): Avoid compiler warnings.
* tools/virsh.c (vshDomainState, vshDomainStateReasonToString):
Likewise.
* src/libvirt_private.syms (domain_conf.h): Export new functions.
* docs/schemas/domainsnapshot.rng: Tighten state definition.
* docs/formatsnapshot.html.in: Document it.
* tests/domainsnapshotxml2xmlout/disk_snapshot.xml: New test.

tools/virsh.c

@@ -14588,6 +14588,8 @@ vshDomainState(vshControl *ctl, virDomainPtr dom, int *reason)
 static const char *
 vshDomainStateToString(int state)
 {
+    /* Can't use virDomainStateTypeToString, because we want to mark
+     * strings for translation.  */
     switch ((virDomainState) state) {
     case VIR_DOMAIN_RUNNING:
         return N_("running");
@@ -14602,6 +14604,7 @@ vshDomainStateToString(int state)
     case VIR_DOMAIN_CRASHED:
         return N_("crashed");
     case VIR_DOMAIN_NOSTATE:
+    default:
         ;/*FALLTHROUGH*/
     }
     return N_("no state");  /* = dom0 state */
@@ -14703,6 +14706,9 @@ vshDomainStateReasonToString(int state, int reason)
             ;
         }
         break;
+
+    default:
+        ;
     }
 
     return N_("unknown");