qemu.tests: fix incorrect test steps in qemu_img.rebase test

qemu-img rebase function used to changes backing_file and backing_file
format for an image file;
In this case, we create snapshots "base->sn1->sn2", then rebase sn2's backing_file
to base, then bootup with sn2 and check sn2's backing_file is base;

Signed-off-by: Xu Tian <xutian@redhat.com>