port.lua: Remove verbose printouts in echo() test.
virtio.lua: Fixes and optimizations.

Fixed ring array indexing out-of-bounds error.

Set flags so the kernel doesn't send us interrupts that we don't need.

Synchronization now checks that we really have new state to push
before executing memory barriers etc.
virtio_vring.h: Fixed error in vring_used structure definition.
virtio_vhost.h: Matched memory alignment more closely to the spec.

Added a (commented out) padding structure that is possibly useful for
chasing a heap corruption bug.
virtio.lua: Removed an unneeded update_vhost_memory_map() call.
buffer.lua: Added missing require() for memory module.