Add VIR_ERR_CONFIG_UNSUPPORTED to fakelibvirt

Commit 867bdedf81533f283aae4de4488d54c254bb7f07 added a test that raises
an exception using the libvirt VIR_ERR_CONFIG_UNSUPPORTED error code but
missed adding that to fakelibvirt, so the test won't work if you don't
have libvirt installed.

This simply adds the constant to fakelibvirt.

Closes-Bug: #1376363

Change-Id: I909608a7f1a8b5ee407b1d01896bc9db1adcee4e
Merge "Add VIR_ERR_CONFIG_UNSUPPORTED to fakelibvirt"