memory: initialize 'fv' in MemoryRegionCache to make Coverity happy

Coverity scan reports multiple false-positive "defects" for the
following series of actions in virtio.c:

  MemoryRegionCache indirect_desc_cache;
  address_space_cache_init_empty(&indirect_desc_cache);
  address_space_cache_destroy(&indirect_desc_cache);

For some reason it's unable to recognize the dependency between 'mrs.mr'
and 'fv' and insists that '!mrs.mr' check in address_space_cache_destroy
may take a 'false' branch, even though it is explicitly initialized to
NULL in the address_space_cache_init_empty():

  *** CID 1522371:  Memory - illegal accesses  (UNINIT)
  /qemu/hw/virtio/virtio.c: 1627 in virtqueue_split_pop()
  1621         }
  1622
  1623         vq->inuse++;
  1624
  1625         trace_virtqueue_pop(vq, elem, elem->in_num, elem->out_num);
  1626     done:
  >>>     CID 1522371:  Memory - illegal accesses  (UNINIT)
  >>>     Using uninitialized value "indirect_desc_cache.fv" when
  >>>     calling "address_space_cache_destroy".
  1627         address_space_cache_destroy(&indirect_desc_cache);
  1628
  1629         return elem;
  1630
  1631     err_undo_map:
  1632         virtqueue_undo_map_desc(out_num, in_num, iov);

  ** CID 1522370:  Memory - illegal accesses  (UNINIT)

Instead of trying to silence these false positive reports in 4
different places, initializing 'fv' as well, as this doesn't result
in any noticeable performance impact.

Signed-off-by: Ilya Maximets <i.maximets@ovn.org>
Message-Id: <20231009104322.3085887-1-i.maximets@ovn.org>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Reviewed-by: David Hildenbrand <david@redhat.com>
Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>