virt: Fix broken indentation

I introduced indentation using tabs instead of spaces in another
commit.  Peter reported the problem, and I failed to fix that
before sending my pull request.

Reported-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Thomas Huth <thuth@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Message-id: 20181212003147.29604-1-ehabkost@redhat.com
Fixes: 95159760 ("virt: Eliminate separate instance_init functions")
Signed-off-by: Eduardo Habkost <ehabkost@redhat.com>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>