vdpa: add vmstate header file

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



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Add the vmstate.h header file to define the structure of vdpa device
status to achieve live migration of cross-vendor vdpa devices.

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