virtio rewrite/reorganization.

Lua code rewritten for clarity. (Broken too, and not yet debugged.)
C headers split into separate virtio and vhost sections.
Compile-time dependency on virtio/vhost headers is removed.
book: Reorganized files & updated Virtio introduction.

The .md chapter introductions moved from src/ into src/doc/
virtio book edits.
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.
port.lua: Updated spam() function to work again.
virtio.lua: Optimize kick() to use VRING_F_NO_NOTIFY.

That is: don't kick the kernel's eventfd if it advertises that this
isn't needed. This should mean that we only kick the eventfd when the
kernel doesn't know there is any data available for processing i.e.
coalesce kicks during bursts of transfers.
virtio.lua: Test with spam() instead of echo().

That way something interesting happens without requiring you to
manually run processes on the Linux side e.g. ifconfig/arp/ping/etc.
virtio_vring.h: Added VRING_F_NO_NOTIFY constant.
vhost: Map a unified memory region with VHOST_SET_MEM_TABLE ioctl.

Previously we registered each chunk of DMA memory as its own region in
this ioctl. However, this hit a Linux/vhost limitation of 64 max
regions. So now we coalesce them all together and nevermind if that
ends up including some other memory too.
port.lua, pci.lua: Print Intel10G-specific statistics.
Merge pull request #72 from SnabbCo/virtio_edit

Rewrite virtio/vhost for clarity.