revert(minimald): revert the 8 MiB guest vsock receive window - #916

Reverts commit b0f07a2 (PR #916), which reverted 4823653 (PR #885).

#885 was reverted before the libkrun fix had been tested in production.
That test has now run and the fix is not sufficient on its own: the
shipped 6d239bc build carries the fill loop and still fails, because
the loop exits on EAGAIN with a partly filled descriptor whenever the
writer is slower than libkrun's reader.

The window is the one measure with an unambiguous result behind it --
0 of 6 failures at 8 MiB against 6 of 6 on the same machine at the
256 KiB default. It raises the skb-depth ceiling from 455 queued
packets to 14,563, which is the bound every observed reset has hit.

This is headroom, not immunity: it does not change the ratio between
the two ceilings, it keeps a normal upload out of the fragmented tail
where the reset happens. It belongs alongside a libkrun fix rather
than instead of one.

Note `SO_VM_SOCKETS_BUFFER_MAX_SIZE` must be raised before
`SO_VM_SOCKETS_BUFFER_SIZE`: vsock_update_buffer_size() clamps the
latter to the former and both default to 256 KiB, so the obvious
ordering silently changes nothing.

Refs: #869