qemu: validate: Reject empty USB disks

Attempting to start qemu with or hotplug an empty 'usb-storage' based
disk results in the following error:

 qemu-system-x86_64: -device {"driver":"usb-storage","bus":"usb.0","port":"2","id":"usb-disk1","removable":true}: drive property not set

Reject such config at validation step and adjust tests.

Signed-off-by: Peter Krempa <pkrempa@redhat.com>
Reviewed-by: Martin Kletzander <mkletzan@redhat.com>