9pfs/virtio: implement msize_limit callback

Add and implement the msize_limit callback for the virtio transport.

This new callback function provides the theoretical maximum 'msize'
value supported by this virtio transport.

The limit is calculated as (VIRTQUEUE_MAX_SIZE - 2) * 4096 bytes,
where 2 virtio descriptors are lost exactly for:

  - 1 descriptor for the original request (typically being small)

  - 1 descriptor as indirect table pointer (when used), which just
    contains a pointer to the separate sglist containing the
    response's actual payload data

And 4096 bytes are assumed as standard page size used by Linux 9p
client. This results in a maximum 'msize' of 4186112 bytes.

Theoretically Linux client could support a much larger size, e.g. by
using multiple consecutive pages per sg entry / descriptor. However
that's currently not the case and unlikely to change any time soon.
And due to recent security issues, let's handle this limit
conservatively until really necessary to be raised.

Link: https://lore.kernel.org/qemu-devel/4c34426bc906e19423e7e2389c419c2e972d9b9b.1781287774.git.qemu_oss@crudebyte.com
Signed-off-by: Christian Schoenebeck <qemu_oss@crudebyte.com>