allow memballoon type of none to desactivate it

allow memballoon type of none to desactivate it

  The balloon device is automatically added to qemu guests if supported,
but it may be useful to desactivate it. The simplest to not change the
existing behaviour is to allow
  <memballoon type="none"/>
as an extra option to desactivate it (it is automatically added if the
memballoon construct is missing for the domain).
The following simple patch just adds the extra option and does not
change the default behaviour but avoid creating a balloon device if
type="none" is used.

* docs/schemas/domain.rng: add the extra type attribute value
* src/conf/domain_conf.c src/conf/domain_conf.h: add the extra enum
  value
* src/qemu/qemu_conf.c: if enum is NONE, don't activate the device,
  i.e. don't pass the args to qemu/kvm