remus: fix incorrect error handling for switch_qemu_logdirty in checkpoint code

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>
xen-unstable changeset:   23202:67911039ac94
xen-unstable date:        Tue Apr 12 13:28:51 2011 +0100