virttest.qemu_vm: use add_cdrom if cd_format is none

In the situation of  cd_format is none, it will use 'ide'
for cdrom, it may be correct in x86, but ppc64 QEMU doesn't
support 'ide'.

So use add_cdrom(), if cd_format is none, to make both arch works.

Signed-off-by: Mike Qiu <qiudayu@linux.vnet.ibm.com>