Libvirt didn't prefix the random number generator backend object alias
with any string thus the device alias and object alias were identical.

To avoid possible problems, rename the alias for the backend object and
tweak tests to comply with the change.
Signed-off-by: NLuyao Huang <lhuang@redhat.com>
Signed-off-by: NPeter Krempa <pkrempa@redhat.com>