vhost: Remove vhost_backend_can_merge() callback

vhost: Remove vhost_backend_can_merge() callback

Checking whether the memory regions are equal is sufficient: if they are
equal, then most certainly the contained fd is equal.

The whole vhost-user memslot handling is suboptimal and overly
complicated. We shouldn't have to lookup a RAM memory regions we got
notified about in vhost_user_get_mr_data() using a host pointer. But that
requires a bigger rework -- especially an alternative vhost_set_mem_table()
backend call that simply consumes MemoryRegionSections.

For now, let's just drop vhost_backend_can_merge().

Message-ID: <20230926185738.277351-3-david@redhat.com>
Acked-by: Stefan Hajnoczi <stefanha@redhat.com>
Reviewed-by: Igor Mammedov <imammedo@redhat.com>
Acked-by: Igor Mammedov <imammedo@redhat.com>
Reviewed-by: Peter Xu <peterx@redhat.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: David Hildenbrand <david@redhat.com>