i2c: virtio:refine the driver of i2c virtualization

- 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.

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

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

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