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.