use better usb config parsing

use better usb config parsing

since we want the usb3 option to be really boolean and not only
'usb3=yes', we have to change the usb json format a little

to not break existing configs for 'usbX: spice', we set the 'host'
option as non-optional and default_key and allow 'spice' as its
content (this also makes the option less ambiguous)

another side effect is that previously accepted multiple 'host='
entries are now forbidden

Signed-off-by: Dominik Csapak <d.csapak@proxmox.com>