vdpa_sim: use kvmalloc to allocate vdpasim->buffer

The next patch will make the buffer size configurable from each
device.
Since the buffer could be larger than a page, we use kvmalloc()
instead of kmalloc().

Acked-by: Jason Wang <jasowang@redhat.com>
Signed-off-by: Stefano Garzarella <sgarzare@redhat.com>
Link: https://lore.kernel.org/r/20201215144256.155342-16-sgarzare@redhat.com


Signed-off-by: Michael S. Tsirkin <mst@redhat.com>