tools: libxc: fix Remus failover regression

commit: c2ba706c "tools/libxc: goto correct label on error paths" by Andrew
Cooper broke Remus in Xen 4.4 or earlier versions that has this commit
backported.

With Remus, this jump essentially discards the current incomplete checkpoint
received by the backup and restore backup from the last complete checkpoint.
This is required for Remus to work and this does not break live migration.  It
has been around since Xen 4.0.

CC: Ian Jackson <ian.jackson@eu.citrix.com>
CC: Ian Campbell <ian.campbell@citrix.com>
CC: Andrew Cooper <andrew.cooper3@citrix.com>
CC: Shriram Rajagopalan <rshriram@cs.ubc.ca>
Signed-off-by: Yang Hongyang <yanghy@cn.fujitsu.com>
Acked-by: Shriram Rajagopalan <rshriram@cs.ubc.ca>
Reviewed-by: Andrew Cooper <andrew.cooper3@citrix.com>
Acked-by: Ian Campbell <ian.campbell@citrix.com>

tools/libxc/xc_domain_restore.c

@@ -1783,20 +1783,29 @@ int xc_domain_restore(xc_interface *xch, int io_fd, uint32_t dom,
 
     if ( pagebuf_get(xch, ctx, &pagebuf, io_fd, dom) ) {
         PERROR("error when buffering batch, finishing");
-        goto out;
+        /*
+         * Remus: discard the current incomplete checkpoint and restore
+         * backup from the last complete checkpoint.
+         */
+        goto finish;
     }
     memset(&tmptail, 0, sizeof(tmptail));
     tmptail.ishvm = hvm;
     if ( buffer_tail(xch, ctx, &tmptail, io_fd, max_vcpu_id, vcpumap,
                      ext_vcpucontext, vcpuextstate_size) < 0 ) {
         ERROR ("error buffering image tail, finishing");
-        goto out;
+        /*
+         * Remus: discard the current incomplete checkpoint and restore
+         * backup from the last complete checkpoint.
+         */
+        goto finish;
     }
     tailbuf_free(&tailbuf);
     memcpy(&tailbuf, &tmptail, sizeof(tailbuf));
 
     goto loadpages;
 
+  /* With Remus: restore from last complete checkpoint */
   finish:
     if ( hvm )
         goto finish_hvm;