i2c: virtio: reallocate memory for each msg buffer

We'd better reallocate memory for each msg buffer to avoid
the memory in stack space be transferred to the backend for
the driver of i2c virtualization.

Change-Id: I6daa58cf819d4f612ae246b5a685898e28fbeb54
Signed-off-by: Xianbin Zhu <xianzhu@codeaurora.org>