qemu: do not overwrite the error in qemuDomainObjExitMonitor
    
    https://bugzilla.redhat.com/show_bug.cgi?id=1196934
    
    When qemu exits during startup, libvirt includes the error from
    /var/log/libvirt/qemu/vm.log in the error message:
    
    $ virsh start test3
    error: Failed to start domain test3
    error: internal error: early end of file from monitor: possible problem:
    2015-02-27T03:03:16.985494Z qemu-kvm: -numa memdev is not supported by
    machine rhel6.5.0
    
    The check for domain liveness added to qemuDomainObjExitMonitor
    in commit dc2fd51f sometimes overwrites this error:
    $ virsh start test3
    error: Failed to start domain test3
    error: operation failed: domain is no longer running
    
    Fix the check to only report an error if there is none set.
    
    Signed-off-by: Luyao Huang <lhuang@redhat.com>
    Signed-off-by: Ján Tomko <jtomko@redhat.com>