virt-libvirt: Error message enhancement

It's possible to install libvirt bits w/o installing kvm or xen.  This
will be reflected in 'virsh capabilities' output, but the VM class just
reports a key error.  Added two specific error logging messages to make
the problem clearer, but continue to raise the original exception.

Signed-off-by: Chris Evich <cevich@redhat.com>