virtio: Reorder fields in 'struct virtqueue'

virtio: Reorder fields in 'struct virtqueue'

Group some variables based on their sizes to reduce hole and avoid padding.
On x86_64, this shrinks the size of 'struct virtqueue'
from 72 to 68 bytes.

It saves a few bytes of memory.

Signed-off-by: Christophe JAILLET <christophe.jaillet@wanadoo.fr>
Message-Id: <8f3d2e49270a2158717e15008e7ed7228196ba02.1676707807.git.christophe.jaillet@wanadoo.fr>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Acked-by: Peter Lafreniere <peter@n8pjl.ca>
Reviewed-by: Stefano Garzarella <sgarzare@redhat.com>