qemu.tests: Add a subtest stub

This testcase which itself is doing nothing is used for dealing with VM(s)
via framework, i.e. create_image, destroy VM(s) etc.

Signed-off-by: Amos Kong <akong@redhat.com>

This patch try to fix all the tab and trailing space problems.

Signed-off-by: Jason Wang <jasowang@redhat.com>