When merging virtio-net buffers using the VIRTIO_NET_F_MRG_RXBUF feature,
the first buffer added to the used ring should indicate the total number
of buffers used to hold the packet. Unfortunately, kvmtool has a number
of issues when constructing these merged buffers:

  - Commit 5131332e3f1a ("kvmtool: convert net backend to support
    bi-endianness") introduced a strange loop counter, which resulted in
    hdr->num_buffers being set redundantly the first time round

  - When adding the buffers to the ring, we actually add them one-by-one,
    allowing the guest to see the header before we've inserted the rest
    of the data buffers...

  - ... which is made worse because we non-atomically increment the
    num_buffers count in the header each time we insert a new data buffer

Consequently, the guest quickly becomes confused in its net rx code and
the whole thing grinds to a halt. This is easily exemplified by trying
to boot a root filesystem over NFS, which seldom succeeds.

This patch resolves the issues by allowing us to insert items into the
used ring without updating the index. Once the full payload has been
added and num_buffers corresponds to the total size, we *then* publish
the buffers to the guest.

Cc: Marc Zyngier <marc.zyngier@arm.com>
Cc: Sasha Levin <sasha.levin@oracle.com>
Signed-off-by: Will Deacon <will.deacon@arm.com>