Fix error reporting for security driver over remote protocol

Fix error reporting for security driver over remote protocol

* qemud/remote.c: Send back the actual libvirt connection error
  rather than formatting a generic error for security driver
  methods
* src/libvirt.c: Fix virDomainGetSecurityLabel, and
  virNodeGetSecurityModel to correctly set the error on
  the virConnectPtr object, and raise a full error rather
  than warning when not supported