Allow virtio_mmio to attach to ofwbus. Qemu places these here on at least
the AArch64 virtual platform with the Linaro UEFI.

Sponsored by:	The FreeBSD Foundation
symlink(7): Note that stat(1) does not follow symlinks by default.

MFC after:	1 week
mdoc: remove EOL whitespace.
Merge remote-tracking branch 'freebsd/master' into hardened/current/master