Inside a virtual machine, writing the RX ring tail pointer may incur a
substantial overhead of processing inside the hypervisor.  Minimise
this overhead by writing the tail pointer once per batch of
descriptors, rather than once per descriptor.

Profiling under qemu-kvm (version 1.6.2) shows that this reduces the
amount of time taken to refill the RX descriptor ring by around 90%.

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