kvm tools: Fix large disk images on 32-bit

Use read()/write() instead of mmap() for virtio block device emulation to
support large disk images on 32-bit.

Reported-and-tested-by: Asias He <asias.hejun@gmail.com>
Tested-by: Cyrill Gorcunov <gorcunov@gmail.com>
Signed-off-by: Pekka Enberg <penberg@kernel.org>