Move the VIRTIO_RING_F_* defines out of virtqueue.h into virtio_config.h

These defines are applicable to userland too, but virtqueue.h contains
the kernel virtqueue interface, and is therefore not usable in userland.

Note that Linux places these defines in virtio_ring.h, but I don't want
the drivers including this header file to keep the VirtIO ring opaque to
everything but the virtqueue.

MFC after:	1 week
Make the feature negotiation code easier to follow

MFC after:	1 week
Force two byte alignment for all control message headers

The header structure consists of two 1-byte elements, but it must always
be describable by a single SG entry. Note for consistency, specify the
alignment everywhere, even if the structure has the appropriate natural
alignment since it contains a uint16_t.

Obtained from:	DragonFlyBSD
MFC after:	1 week