virtq: Use hardware memory barrier in "call"

This fixes a subtle bug where the guest can become stuck due to
missing an interrupt. This happens with e.g. Linux kernel guests that
use interrupts and not e.g. DPDK guests that are polling.

See details on snabb-devel:
https://groups.google.com/d/msg/snabb-devel/8oGjqlSkiqw/4Yi9CVEdjD0J

src/lib/virtio/virtq.lua

@@ -101,9 +101,10 @@ end
 -- Prepared argument for writing a 1 to an eventfd.
 local eventfd_one = ffi.new("uint64_t[1]", {1})
 
-function VirtioVirtq:signal_used()
+function VirtioVirtq:signal_used ()
    if self.virtq.used.idx ~= self.used then
       self.virtq.used.idx = self.used
+      C.full_memory_barrier()
       if band(self.virtq.avail.flags, C.VRING_F_NO_INTERRUPT) == 0 then
          C.write(self.callfd, eventfd_one, 8)
       end