kvm tools: Make raw block device work

Previously, we use mmaped host root partition as guest's root
filesystem. As virtio-9p based root filesystem is supported,
mmaped host root partition approach is not used anymore.

It is useful to use raw block device as guest's disk backend for some
user. e.g. bypass host's fs layer.

This patch makes raw block device work as disk image, user can do
read/write on raw block device, by using DISK_IMAGE_REGULAR instead of
DISK_IMAGE_MMAP for block device

Signed-off-by: Asias He <asias.hejun@gmail.com>
Signed-off-by: Pekka Enberg <penberg@kernel.org>