Use floppy disk and a second CDROM on KVM

Hi all,
this patch will add 3 new KVM parameters and a new option.

New Parameters:
 - floppy_image_path = "" -> Specify the floppy image to load as
floppy disk.
 - cdrom2_image_path = "" -> Specify a second cdrom image to load on
the system (note: this in not intended to be used as a boot device. To
boot the system from cdrom you must use the "cdrom_image_path"
parameter as always).
 - cdrom_disk_type = "" -> it can be one of the kvm supported types as
"ide,scsi,paravirtual,ecc". I introduced this optional parameter to
make possible to specify a different virtual device for cdroms. It is
useful if you want to install a windows system

New option for "boot_device" parameter:
 -  "floppy": with this value you should be able to boot a KVM
instance from floppy image.

Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: Iustin Pop <iustin@google.com>