If the debugging is enabled, the virCommand subsystem catches debug
messages in the command output as well. In that case, we can't assume
the string corresponding to command's stdout will start with specific
prefix. But the prefix can be moved deeper in the string. This bug
shows itself when parsing dnsmasq output:

2012-12-06 11:18:11.445+0000: 18491: error :
dnsmasqCapsSetFromBuffer:664 : internal error cannot parse
/usr/sbin/dnsmasq version number in '2012-12-06 11:11:02.232+0000:
18492: debug : virFileClose:72 : Closed fd 22'

We can clearly see that the output of dnsmasq --version
doesn't start with expected "Dnsmasq version " string but a libvirt
debug output.