virtio-balloon: kzalloc the vb struct

Zero all the vb fields at alloaction, so that we don't need to
zero-initialize each field one by one later.

Signed-off-by: Wei Wang <wei.w.wang@intel.com>
Cc: Michael S. Tsirkin <mst@redhat.com>
Cc: Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>