vhost-vdpa: add uAPI for device buffer

virt inclusion
category: feature
bugzilla: https://gitee.com/openeuler/kernel/issues/I86ITO



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These new ioctl add support for saving/loading device status from
usersapce.

When vhost-vdpa device start migration, VMM need to call these ioctl to
save/load device status of vhost-vdpa devices if these ops is
implemented.

Signed-off-by: jiangdongxu <jiangdongxu1@huawei.com>