balloon: Use only 'pc-dimm' type dimm for ballooning

For now there are only two dimm's: pc-dimm and nvdimm. This patch is
actually needed to disable ballooning on nvdimm. But, to avoid future
bugs, instead of disallowing nvdimm, we allow only pc-dimm. So, if
someone adds new dimm which should be balloon-able, then this ability
should be explicitly specified here.

Why ballooning for nvdimm should be disabled for now:

NVDIMM for now is planned to use as a backing store for DAX filesystem
in the guest and thus this memory is excluded from guest memory
management and LRUs.

In this case libvirt running QEMU along with configured balloon almost
immediately inflates balloon and effectively kill the guest as
qemu counts nvdimm as part of the ram.

Counting dimm devices as part of the ram for ballooning was started from
commit 463756d03:
 virtio-balloon: Fix balloon not working correctly when hotplug memory

Signed-off-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
Signed-off-by: Denis V. Lunev <den@openvz.org>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
