virtio_mmio: add new mmio interface

Change-Id: I2ca51d98e337d1e69a8e68d78ef0cfe33dc0d308
Reviewed-on: https://review.haiku-os.org/c/haiku/+/3979
Tested-by: Commit checker robot <no-reply+buildbot@haiku-os.org>
Reviewed-by: Jérôme Duval <jerome.duval@gmail.com>