virtio: make the write_config buffer argument const

No functional change intended, but noticed that we could add const here
while adding linuxkpi support for virtio.

Reviewed By: bryanv, imp
Differential Revision: https://reviews.freebsd.org/D32370

(cherry picked from commit 6c4f95161d6e38ba3fd50ca1cceee3110cbfa02e)