Add a hook to the error reporting APIs to allow specific
error messages to be filtered out. Wire up libvirtd to
remove VIR_ERR_NO_DOMAIN & similar error codes from the
logs. They are still logged at DEBUG level.

* daemon/libvirtd.c: Filter VIR_ERR_NO_DOMAIN and friends
* src/libvirt_private.syms, src/util/virterror.c,
  src/util/virterror_internal.h: Hook for changing error
  reporting level