virtio: Support transports which can specify the vring alignment

virtio: Support transports which can specify the vring alignment

Support virtio transports which can specify the vring alignment
(ie where the guest communicates this to the host) by providing
a new virtio_queue_set_align() function. (The default alignment
remains as before.)

Transports which wish to make use of this must set the
has_variable_vring_alignment field in their VirtioBusClass
struct to true; they can then change the alignment via
virtio_queue_set_align().

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Message-id: 1373977512-28932-5-git-send-email-peter.maydell@linaro.org