[gve] Allow for out-of-order buffer consumption

[gve] Allow for out-of-order buffer consumption

We currently assume that the buffer index is equal to the descriptor
ring index, which is correct only for in-order queues.

Out-of-order queues will include a buffer tag value that is copied
from the descriptor to the completion.  Redefine the data buffers as
being indexed by this tag value (rather than by the descriptor ring
index), and add a circular ring buffer to allow for tags to be reused
in whatever order they are released by the hardware.

Signed-off-by: Michael Brown <mcb30@ipxe.org>