i2c: refine the driver of i2c virtualization

1. there's no need to allocate buffer for virtio_i2c_req every
   time when it comes to data transmission, we can prepare this
   buffer at probe stage.

2. we need to add some input parameter checking for avoiding
   security risks.

3. refine the code to reduce the number of judgement statements.

Change-Id: Idb6ae8bb632c1a3032c6ba04de6ebb460f1e1a5c
Signed-off-by: Xianbin Zhu <xianzhu@codeaurora.org>