vdpa_sim: make vdpasim->buffer size configurable

stable inclusion
from stable-v5.11
commit da7af696
category: feature
bugzilla: https://gitee.com/openeuler/kernel/issues/I5WXCZ
CVE: NA

Reference: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=da7af6967c6e9815f8da60a8db1d0fe35b8e97b9



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Allow each device to specify the size of the buffer allocated
in vdpa_sim.

Acked-by: Jason Wang <jasowang@redhat.com>
Signed-off-by: Stefano Garzarella <sgarzare@redhat.com>
Link: https://lore.kernel.org/r/20201215144256.155342-17-sgarzare@redhat.com


Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Pengyuan Zhao <zhaopengyuan@hisilicon.com>