snapshot: add qemu capability for 'transaction' command

We need a capability bit to gracefully error out if some of the
additions in future patches can't be implemented by the running qemu.

* src/qemu/qemu_capabilities.h (QEMU_CAPS_TRANSACTION): New cap.
* src/qemu/qemu_capabilities.c (qemuCaps): Name it.
* src/qemu/qemu_monitor_json.c (qemuMonitorJSONCheckCommands): Set
it.

src/qemu/qemu_capabilities.c

@@ -155,6 +155,7 @@ VIR_ENUM_IMPL(qemuCaps, QEMU_CAPS_LAST,
               "system_wakeup",
               "scsi-disk.channel",
               "scsi-block",
+              "transaction",
     );
 
 struct qemu_feature_flags {

src/qemu/qemu_capabilities.h

@@ -123,6 +123,7 @@ enum qemuCapsFlags {
     QEMU_CAPS_WAKEUP             = 86, /* system_wakeup monitor command */
     QEMU_CAPS_SCSI_DISK_CHANNEL  = 87, /* Is scsi-disk.channel available? */
     QEMU_CAPS_SCSI_BLOCK         = 88, /* -device scsi-block */
+    QEMU_CAPS_TRANSACTION        = 89, /* transaction monitor command */
 
     QEMU_CAPS_LAST,                   /* this must always be the last item */
 };

src/qemu/qemu_monitor_json.c

@@ -929,6 +929,9 @@ qemuMonitorJSONCheckCommands(qemuMonitorPtr mon,
 
         if (STREQ(name, "system_wakeup"))
             qemuCapsSet(qemuCaps, QEMU_CAPS_WAKEUP);
+
+        if (STREQ(name, "transaction"))
+            qemuCapsSet(qemuCaps, QEMU_CAPS_TRANSACTION);
     }
 
     ret = 0;