virtio: limit ring size to a maximum of 128 descriptor entries

QEMU commit c9b7d9ec21 "virtio: increase virtqueue size for virtio-scsi and
virtio-blk" increased the number of queue descriptors from 128 to 256 which
caused OpenBIOS to fail when booting from a virtio-blk device under
qemu-system-ppc due to a memory overflow.

Update the virtio-blk driver so that it uses a maximum of 128 descriptor
entries (the previous value) to fix the issue, and also ensure that any
further increases in virtqueue size will not cause the same failure.

Note that this commit also fixes the vring_area size calculation which was
incorrectly based upon the number of virtqueues, and not the number of
descriptor entries.

Signed-off-by: Mark Cave-Ayland <mark.cave-ayland@ilande.co.uk>