Set the page size in the virtio-mmio driver. Some backends, e.g QEMU, assume
a 1 byte page size until told otherwise.

Sponsored by:	The FreeBSD Foundation


git-svn-id: svn+ssh://svn.freebsd.org/base/head@276409 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f