backup: Add new domain:checkpoint access control

Creating a checkpoint does not modify guest-visible state,
but does modify host resources.  Rather than reuse existing
domain:write, domain:block_write, or domain:snapshot access
controls, it seems better to introduce a new access control
specific to tasks related to checkpoints and incremental
backups of guest disk state.
Signed-off-by: NEric Blake <eblake@redhat.com>
Reviewed-by: NDaniel P. Berrangé <berrange@redhat.com>

src/access/viraccessperm.c

@@ -39,7 +39,8 @@ VIR_ENUM_IMPL(virAccessPermDomain,
               "getattr", "read", "write", "read_secure",
               "start", "stop", "reset",
               "save", "delete",
-              "migrate", "snapshot", "suspend", "hibernate", "core_dump", "pm_control",
+              "migrate", "checkpoint", "snapshot", "suspend", "hibernate",
+              "core_dump", "pm_control",
               "init_control", "inject_nmi", "send_input", "send_signal",
               "fs_trim", "fs_freeze",
               "block_read", "block_write", "mem_read",

src/access/viraccessperm.h

@@ -186,6 +186,12 @@ typedef enum {
      */
     VIR_ACCESS_PERM_DOMAIN_MIGRATE,   /* Host migration */
 
+    /**
+     * @desc: Checkpoint domain
+     * @message: Checkpointing domain requires authorization
+     */
+    VIR_ACCESS_PERM_DOMAIN_CHECKPOINT,  /* Checkpoint disks */
+
     /**
      * @desc: Snapshot domain
      * @message: Snapshotting domain requires authorization