If there is no driver for a URI we report

  "no hypervisor driver available"

This is bad because not all virt drivers are hypervisors (ie container
based virt).

If there is no driver support for an API we report

  "this function is not supported by the hypervisor"

This is bad for the same reason, and additionally because it is
also used for the network, interface & storage drivers.

* src/util/virterror.c: Improve error messages