virtio-balloon: Avoid using the word 'report' when referring to free page hinting

It can be confusing to have multiple features within the same driver that
are using the same verbage. As such this patch is creating a union of
free_page_report_cmd_id with free_page_hint_cmd_id so that we can clean-up
the userspace code a bit in terms of readability while maintaining the
functionality of legacy code.

Signed-off-by: Alexander Duyck <alexander.h.duyck@linux.intel.com>
Link: https://lore.kernel.org/r/20200415174318.13597.99753.stgit@localhost.localdomain


Signed-off-by: Michael S. Tsirkin <mst@redhat.com>