virttest.qemu_devices: Add support for image, cdrom, floppy and usb handling

This patchset adds support for image handling to qemu_devices. It (hopefully) supports all current features plus it fixes couple of existing bugs.

The only difference should be names of disks, which doesn't follow the previous convention, but they use the autotest object name. In test you can use `VM.devices.get()` to get the image and see what parameters it uses.