virtio-balloon: Implement support for page poison reporting feature

virtio-balloon: Implement support for page poison reporting feature

We need to make certain to advertise support for page poison reporting if
we want to actually get data on if the guest will be poisoning pages.

Add a value for reporting the poison value being used if page poisoning is
enabled in the guest. With this we can determine if we will need to skip
free page reporting when it is enabled in the future.

The value currently has no impact on existing balloon interfaces. In the
case of existing balloon interfaces the onus is on the guest driver to
reapply whatever poison is in place.

When we add free page reporting the poison value is used to determine if
we can perform in-place page reporting. The expectation is that a reported
page will already contain the value specified by the poison, and the
reporting of the page should not change that value.

Acked-by: David Hildenbrand <david@redhat.com>
Signed-off-by: Alexander Duyck <alexander.h.duyck@linux.intel.com>
Message-Id: <20200527041400.12700.33251.stgit@localhost.localdomain>