hw/core/machine: move compatibility flags for VirtIO-net USO to machine 8.1

hw/core/machine: move compatibility flags for VirtIO-net USO to machine 8.1

Migration from an 8.2 or 9.0 binary to an 8.1 binary with machine
version 8.1 can fail with:

> kvm: Features 0x1c0010130afffa7 unsupported. Allowed features: 0x10179bfffe7
> kvm: Failed to load virtio-net:virtio
> kvm: error while loading state for instance 0x0 of device '0000:00:12.0/virtio-net'
> kvm: load of migration failed: Operation not permitted

The series

53da8b5a99 virtio-net: Add support for USO features
9da1684954 virtio-net: Add USO flags to vhost support.
f03e0cf63b tap: Add check for USO features
2ab0ec3121 tap: Add USO support to tap device.

only landed in QEMU 8.2, so the compatibility flags should be part of
machine version 8.1.

Moving the flags unfortunately breaks forward migration with machine
version 8.1 from a binary without this patch to a binary with this
patch.

Fixes: 53da8b5a99 ("virtio-net: Add support for USO features")
Signed-off-by: Fiona Ebner <f.ebner@proxmox.com>
Reviewed-by: Fabiano Rosas <farosas@suse.de>
Acked-by: Jason Wang <jasowang@redhat.com>
Reviewed-by: Peter Xu <peterx@redhat.com>
Signed-off-by: Fabiano Rosas <farosas@suse.de>
(cherry picked from commit 9710401276a0eb2fc6d467d9abea1f5e3fe2c362)
Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>