qemu: Remove explicit references to guest image paths

Since now the virt API can figure out the appropriate
guest image from the test parameters, and the --guest-image-path
command line option passed to the avocado runner.

Signed-off-by: Lucas Meneghel Rodrigues <lmr@redhat.com>
qemu: Call previous setup() methods

The Virt Test class now can restore the base image files
to their pristine conditions, depending on command line
options. So, let's call the previous setup methods before
implementing our own logic.

Signed-off-by: Lucas Meneghel Rodrigues <lmr@redhat.com>