kvm tools: Tune up ops in 'struct disk_image_operations'

Make read/write ops in 'struct disk_image_operations'
always return the number of bytes read/written and close/flush
ops return int.

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