remus: fix incorrect error handling for switch_qemu_logdirty in checkpoint code

c/s 22275: "tools: cleanup domain save switch_qemu_logdirty callback"
introduced a whole bunch of error code fixups. In the process, it also
ended up treating the success return code (0) from
switch_qemu_logdirty as an error and vice versa.

Signed-off-by: Shriram Rajagopalan <rshriram@cs.ubc.ca>

tools/python/xen/lowlevel/checkpoint/libcheckpoint.c

@@ -185,7 +185,7 @@ int checkpoint_start(checkpoint_state* s, int fd,
     hvm = s->domtype > dt_pv;
     if (hvm) {
        flags |= XCFLAGS_HVM;
-       if (!switch_qemu_logdirty(s, 1))
+       if (switch_qemu_logdirty(s, 1))
            return -1;
     }