Avoid useless libvirt error log produced by featuretests

When running feature tests, we get bunch of error messages as below:

*** Running feature tests ***
libvirt: Domain Config error : internal error unknown protocol type 'http'
libvirt: Domain Config error : internal error unknown protocol type 'https'
libvirt: Domain Config error : internal error unknown protocol type 'ftp'
libvirt: Domain Config error : internal error unknown protocol type 'ftps'
libvirt: Domain Config error : internal error unknown protocol type 'tftp'
Cannot find '' in path: No such file or directory
*** Feature tests completed ***

By replacing default error handler of libvirt, this patch succeeded to
avoid the errors. After Featuretest, the default error handler will be restored.
The default error handler (in libvirtmod.so) just do one thing, it prints the
error message to stderr before we catch the exception in python.

Independent of the error, it will be logged into kimchi error log file.
That way we can easily debug the system.

Signed-off-by: apporc <appleorchard2000@gmail.com>
Signed-off-by: Aline Manera <alinefm@br.ibm.com>