virtio-balloon: Prevent guest from starting a report when we didn't request one

Based on code review it appears possible for the driver to force the device
out of a stopped state when hinting by repeating the last ID it was
provided.

Prevent this by only allowing a transition to the start state when we are
in the requested state. This way the driver is only allowed to send one
descriptor that will transition the device into the start state. All others
will leave it in the stop state once it has finished.

Fixes: c13c4153 ("virtio-balloon: VIRTIO_BALLOON_F_FREE_PAGE_HINT")
Acked-by: David Hildenbrand <david@redhat.com>
Signed-off-by: Alexander Duyck <alexander.h.duyck@linux.intel.com>
Message-Id: <20200720175115.21935.99563.stgit@localhost.localdomain>
Cc: qemu-stable@nongnu.org
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>