docs: driver-api: virtio: slightly reword virtqueues allocation paragraph

"It's at this stage that" means "At this point", so use the latter as it
is more effective.

Signed-off-by: Bagas Sanjaya <bagasdotme@gmail.com>
Message-Id: <20221220095828.27588-3-bagasdotme@gmail.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>