Quiet down error reporting from libvirt.

On a system where ``qemu:///system`` is not writable by the user running
salt, the ``kvm_hyper`` module will report a lot of noise due to
permission denials. Registering an error handler will prevent those
messages from displaying in salt output unless the log level is set to
debug. It will be clear to users that the 'virtual' module did not load
from existing log output. The normal action to debug module load misses
is to use debug logging to resolve load issues. I believe this will not
lead to surprising behavior in the intenional use cases because the use
of debug logging will show the libvirt errors. I also believe that
quieting down the logs will be less confusing when users have no
intention of using ``virtual`` and have probably not configured it
correctly, but it was installed on their system.
Merge pull request #3943 from dcolish/cleanup-kvm_hyper-error-reports

Quiet down error reporting from libvirt.