libxl: remove qemu default devices for upstream qemu

Remove default devices created by qemu. Qemu will create only devices
defined by xen, since the devices not defined by xen are not usable.
Remove deleting of empty floppy no more needed with nodefault.

Changes from v1:
Fix multi-line comment style

Signed-off-by: Fabio Fantoni <fabio.fantoni@m2r.biz>