If a VM has an attached CDROM, `backing` attribute of the CDROM will
be defined.
As a result, we cannot just loop `vm_obj.config.hardware.device` and check
for the existance of the attribute to decide if the entry is a
harddrive.
Instead, we check the type of the device, and only keep the
`vim.vm.device.VirtualDisk` disk.
This issue is actually breaking our test-suite with regular environment
because we keep a Fedora ISO attached to the VM.
