virtio-vsock: add max payload size config field

[lauralt]

Added a new field to the vsock device config space that
is limiting the size of the packet payload. This way
the driver is not allowed to allocate huge buffers, and
potentially fill up the entire memory.
Also defined a new feature bit for this, VIRTIO_VSOCK_F_SIZE_MAX,
in order to keep backwards compatibility.

https://lists.oasis-open.org/archives/virtio-comment/202206/msg00030.html

[cohuck]

BALLOT CREATED AT URL: https://www.oasis-open.org/committees/ballot.php?id=3703