qemu: Don't report false errors in migration protocol v2
    
    Finish is the final state in v2 of our migration protocol. If something
    fails, we have no option to abort the migration and resume the original
    domain. Non fatal errors (such as failure to start guest CPUs or make
    the domain persistent) has to be treated as success. Keeping the domain
    running while reporting the failure was just asking for trouble.
    
    Signed-off-by: Jiri Denemark <jdenemar@redhat.com>