qemu_qtree: Fix the problem with attached cdroms in multi_disk

Previously cdroms were not taken into account while checking
disk params thus VMs with cdroms failed with additional
devices in qtree but not in params.

Now they are skipped from params detection. As this test is not
focused on cdroms it doesn't check the cdrom params and it doesn't
require the cdrom to be present.

Signed-off-by: Lukáš Doktor <ldoktor@redhat.com>