sys: Remove $FreeBSD$: two-line .h pattern

Remove /^\s*\*\n \*\s+\$FreeBSD\$$\n/

virtio: Silence a -Wunused warning

Remove virtio_swap_endian(). htole*() are nops on little-endian
platforms. No functional change intended.

MFC after: 1 week
Reviewed by: corvink, jhb
Differential Revision: https://reviews.freebsd.org/D37298

bhyve: virtio shares definitions between sys/dev/virtio

Definitions inside usr.sbin/bhyve/virtio.h are thrown away.
Definitions in sys/dev/virtio are used instead.

This reduces code duplication.

Sponsored by: The FreeBSD Foundation
Reviewed by: grehan
Approved by: philip (mentor)
Differential Revision: https://reviews.freebsd.org/D29084

virtio: Add VirtIO PCI modern (V1) support

Use the existing legacy PCI driver as the basis for shared code
between the legacy and modern PCI drivers. The existing virtio_pci
kernel module will contain both the legacy and modern drivers.

Changes to the virtqueue and each device driver (network, block, etc)
for V1 support come in later commits.

Update the MMIO driver to reflect the VirtIO bus method changes, but
the modern compliance can be improved on later.

Note that the modern PCI driver requires bus_map_resource() to be
implemented, which is not the case on all archs.

The hw.virtio.pci.transitional tunable default value is zero so
transitional devices will continue to be driven via the legacy
driver.

Reviewed by: grehan (mentor)
Differential Revision: https://reviews.freebsd.org/D27856