kvm tools: Return correct values from disk IOV functions

Currently read/write IOV functions return an incorrect
value instead of the amount of bytes read/written.

This incorrect value may cause errors within the virtio layer.

Return correct amount of bytes read/written from _iov functions.

[ penberg@kernel.org: don't use 'inline' for out-of-line functions ]
Signed-off-by: Sasha Levin <levinsasha928@gmail.com>
Signed-off-by: Pekka Enberg <penberg@kernel.org>