Build VirtIO modules on all architectures

Currently only amd64, i386 and powerpc build VirtIO modules, yet all other
architectures have at least one kernel configuration that includes the
transport drivers, and so they lack drivers for all the devices they don't
statically compile into the kernel. Instead, enable the build everywhere so all
architectures have the full set of device drivers available.

Reviewed by: bryanv (earlier version), imp (earlier version)
Differential Revision: https://reviews.freebsd.org/D28058