vhost: dirty log should be per backend type

vhost: dirty log should be per backend type

There could be a mix of both vhost-user and vhost-kernel clients
in the same QEMU process, where separate vhost loggers for the
specific vhost type have to be used. Make the vhost logger per
backend type, and have them properly reference counted.

Suggested-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Si-Wei Liu <si-wei.liu@oracle.com>
Message-Id: <1710448055-11709-1-git-send-email-si-wei.liu@oracle.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>