It's been marked as deprecated in QEMU v2.6.0 already, so really nobody
should use the legacy "ivshmem" device anymore (but use ivshmem-plain or
ivshmem-doorbell instead). Time to remove the deprecated device now.

Belatedly also update a mention of the deprecated "ivshmem" in the file
docs/specs/ivshmem-spec.txt to "ivshmem-doorbell". Missed in commit
5400c02b ("ivshmem: Split ivshmem-plain, ivshmem-doorbell off ivshmem").

Signed-off-by: Thomas Huth <thuth@redhat.com>
Reviewed-by: Markus Armbruster <armbru@redhat.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>