xen: ensure /usr/sbin/xend exists before checking status
    
    With xend on the way out, installations may not even have
    /usr/sbin/xend, which results in the following error when the
    drivers are probed
    
    2014-04-28 18:21:19.271+0000: 22129: error : virCommandWait:2426 :
    internal error: Child process (/usr/sbin/xend status) unexpected exit
    status 127: libvirt:  error : cannot execute binary /usr/sbin/xend:
    No such file or directory
    
    Check for existence of /usr/sbin/xend before trying to run it with
    the 'status' option.